THE EIGHTH COMMANDMENT
You shall not bear false witness against your neighbor. [EX 20:16; Cf. Deut 5:20]
It was said to the men of old, "You shall not swear falsely, but shall perform to the Lord what you have sworn." [Mt 5:33]
2464 The eighth commandment forbids misrepresenting the truth in our relations with others. This moral prescription flows from the vocation of the holy people to bear witness to their God who is the truth and wills the truth. Offenses against the truth express by word or deed a refusal to commit oneself to moral uprightness: they are fundamental infidelities to God and, in this sense, they undermine the foundations of the covenant.
I. LIVING IN THE TRUTH
2465 The Old Testament attests that God is the source of all truth. His Word is truth. His Law is truth. His "faithfulness endures to all generations." [PS 119:90; Cf. Prov 8:7; 2 Sam 7:28; PS 119:142; Lk 1:50] Since God is "true," the members of his people are called to live in the truth. [Rom 3:4; Cf. PS 119:30] 
2466 In Jesus Christ, the whole of God's truth has been made manifest. "Full of grace and truth," he came as the "light of the world," he is the Truth. [Jn 1:14; 8:12; Cf. 14:6] "Whoever believes in me may not remain in darkness." [Jn 12:46] The disciple of Jesus continues in his word so as to know "the truth [that] will make you free" and that sanctifies. [Jn 8:32; Cf. 17:17] To follow Jesus is to live in "the Spirit of truth," whom the Father sends in his name and who leads "into all the truth." [Jn 16:13] To his disciples Jesus teaches the unconditional love of truth: "Let what you say be simply 'Yes or No.'" [Mt 5:37] 
2467 Man tends by nature toward the truth. He is obliged to honor and bear witness to it: "It is in accordance with their dignity that all men, because they are persons... are both impelled by their nature and bound by a moral obligation to seek the truth, especially religious truth. They are also bound to adhere to the truth once they come to know it and direct their whole lives in accordance with the demands of truth." [DH 2 § 2] 
2468 Truth as uprightness in human action and speech is called truthfulness, sincerity, or candor. Truth or truthfulness is the virtue which consists in showing oneself true in deeds and truthful in words, and in guarding against duplicity, dissimulation, and hypocrisy. 
2469 "Men could not live with one another if there were not mutual confidence that they were being truthful to one another." [St. Thomas Aquinas, STh II-II 109, 3 ad 1] The virtue of truth gives another his just due. Truthfulness keeps to the just mean between what ought to be expressed and what ought to be kept secret: it entails honesty and discretion. In justice, "as a matter of honor, one man owes it to another to manifest the truth." [St. Thomas Aquinas, STh II-II, 109, 3, corp. art] 
2470 The disciple of Christ consents to "live in the truth," that is, in the simplicity of a life in conformity with the Lord's example, abiding in his truth. "If we say we have fellowship with him while we walk in darkness, we lie and do not live according to the truth." [1 Jn 1:6]
II. TO BEAR WITNESS TO THE TRUTH
2471 Before Pilate, Christ proclaims that he "has come into the world, to bear witness to the truth." [Jn 18:37] The Christian is not to "be ashamed then of testifying to our Lord." [2 Tim 1:8] In situations that require witness to the faith, the Christian must profess it without equivocation, after the example of St. Paul before his judges. We must keep "a clear conscience toward God and toward men." [Acts 24:16] 
2472 The duty of Christians to take part in the life of the Church impels them to act as witnesses of the Gospel and of the obligations that flow from it. This witness is a transmission of the faith in words and deeds. Witness is an act of justice that establishes the truth or makes it known. [Cf. Mt 18:16] [863, 905, 1807]
All Christians by the example of their lives and the witness of their word, wherever they live, have an obligation to manifest the new man which they have put on in Baptism and to reveal the power of the Holy Spirit by whom they were strengthened at Confirmation. [AG 11]
2473 Martyrdom is the supreme witness given to the truth of the faith: it means bearing witness even unto death. The martyr bears witness to Christ who died and rose, to whom he is united by charity. He bears witness to the truth of the faith and of Christian doctrine. He endures death through an act of fortitude. "Let me become the food of the beasts, through whom it will be given me to reach God." [St. Ignatius of Antioch, Ad Rom. 4, 1 SCh 10, 110.]
2474 The Church has painstakingly collected the records of those who persevered to the end in witnessing to their faith. These are the acts of the Martyrs. They form the archives of truth written in letters of blood:
Neither the pleasures of the world nor the kingdoms of this age will be of any use to me. It is better for me to die [in order to unite myself] to Christ Jesus than to reign over the ends of the earth. I seek him who died for us; I desire him who rose for us. My birth is approaching... [St. Ignatius of Antioch, Ad Rom. 6, 1-2 SCh 10, 114] 
I bless you for having judged me worthy from this day and this hour to be counted among your martyrs.... You have kept your promise, God of faithfulness and truth. For this reason and for everything, I praise you, I bless you, I glorify you through the eternal and heavenly High Priest, Jesus Christ, your beloved Son. Through him, who is with you and the Holy Spirit, may glory be given to you, now and in the ages to come. Amen. [Martyrium Polycarpi 14,2-3 PG 5,1040; SCh 10,228]
III. OFFENSES AGAINST TRUTH
2475 Christ's disciples have "put on the new man, created after the likeness of God in true righteousness and holiness." [Eph 4:24] By "putting away falsehood," they are to "put away all malice and all guile and insincerity and envy and all slander." [Eph 4:25; 1 Pet 2:1]
2476 False witness and perjury. When it is made publicly, a statement contrary to the truth takes on a particular gravity. In court it becomes false witness. [Cf. Prov 19:9] When it is under oath, it is perjury. Acts such as these contribute to condemnation of the innocent, exoneration of the guilty, or the increased punishment of the accused. [Cf. Prov 18:5] They gravely compromise the exercise of justice and the fairness of judicial decisions. 
2477 Respect for the reputation of persons forbids every attitude and word likely to cause them unjust injury. [Cf. CIC, can. 220] He becomes guilty:
- of rash judgment who, even tacitly, assumes as true, without sufficient foundation, the moral fault of a neighbor;
- of detraction who, without objectively valid reason, discloses another's faults and failings to persons who did not know them; [Cf. Sir 21:28]
- of calumny who, by remarks contrary to the truth, harms the reputation of others and gives occasion for false judgments concerning them.
2478 To avoid rash judgment, everyone should be careful to interpret insofar as possible his neighbor's thoughts, words, and deeds in a favorable way:
Every good Christian ought to be more ready to give a favorable interpretation to another's statement than to condemn it. But if he cannot do so, let him ask how the other understands it. And if the latter understands it badly, let the former correct him with love. If that does not suffice, let the Christian try all suitable ways to bring the other to a correct interpretation so that he may be saved. [St. Ignatius of Loyola, Spiritual Exercises, 22]
2479 Detraction and calumny destroy the reputation and honor of one's neighbor. Honor is the social witness given to human dignity, and everyone enjoys a natural right to the honor of his name and reputation and to respect. Thus, detraction and calumny offend against the virtues of justice and charity. 
2480 Every word or attitude is forbidden which by flattery, adulation, or complaisance encourages and confirms another in malicious acts and perverse conduct. Adulation is a grave fault if it makes one an accomplice in another's vices or grave sins. Neither the desire to be of service nor friendship justifies duplicitous speech. Adulation is a venial sin when it only seeks to be agreeable, to avoid evil, to meet a need, or to obtain legitimate advantages.
2481 Boasting or bragging is an offense against truth. So is irony aimed at disparaging someone by maliciously caricaturing some aspect of his behavior.
2482 "A lie consists in speaking a falsehood with the intention of deceiving." [St. Augustine, De mendacio 4, 5: PL 40: 491] The Lord denounces lying as the work of the devil: "You are of your father the devil,... there is no truth in him. When he lies, he speaks according to his own nature, for he is a liar and the father of lies." [Jn 8:44] 
2483 Lying is the most direct offense against the truth. To lie is to speak or act against the truth in order to lead someone into error. By injuring man's relation to truth and to his neighbor, a lie offends against the fundamental relation of man and of his word to the Lord.
2484 The gravity of a lie is measured against the nature of the truth it deforms, the circumstances, the intentions of the one who lies, and the harm suffered by its victims. If a lie in itself only constitutes a venial sin, it becomes mortal when it does grave injury to the virtues of justice and charity. 
2485 By its very nature, lying is to be condemned. It is a profanation of speech, whereas the purpose of speech is to communicate known truth to others. The deliberate intention of leading a neighbor into error by saying things contrary to the truth constitutes a failure in justice and charity. The culpability is greater when the intention of deceiving entails the risk of deadly consequences for those who are led astray. 
2486 Since it violates the virtue of truthfulness, a lie does real violence to another. It affects his ability to know, which is a condition of every judgment and decision. It contains the seed of discord and all consequent evils. Lying is destructive of society; it undermines trust among men and tears apart the fabric of social relationships. 
2487 Every offense committed against justice and truth entails the duty of reparation, even if its author has been forgiven. When it is impossible publicly to make reparation for a wrong, it must be made secretly. If someone who has suffered harm cannot be directly compensated, he must be given moral satisfaction in the name of charity. This duty of reparation also concerns offenses against another's reputation. This reparation, moral and sometimes material, must be evaluated in terms of the extent of the damage inflicted. It obliges in conscience. [1459, 2412]
IV. RESPECT FOR THE TRUTH
2488 The right to the communication of the truth is not unconditional. Everyone must conform his life to the Gospel precept of fraternal love. This requires us in concrete situations to judge whether or not it is appropriate to reveal the truth to someone who asks for it. 
2489 Charity and respect for the truth should dictate the response to every request for information or communication. The good and safety of others, respect for privacy, and the common good are sufficient reasons for being silent about what ought not be known or for making use of a discreet language. The duty to avoid scandal often commands strict discretion. No one is bound to reveal the truth to someone who does not have the right to know it. [Cf. Sir 27:16; Prov 25:9-10] 
2490 The secret of the sacrament of reconciliation is sacred, and cannot be violated under any pretext. "The sacramental seal is inviolable; therefore, it is a crime for a confessor in any way to betray a penitent by word or in any other manner or for any reason." [CIC, Can. 983 § 1] 
2491 Professional secrets - for example, those of political office holders, soldiers, physicians, and lawyers - or confidential information given under the seal of secrecy must be kept, save in exceptional cases where keeping the secret is bound to cause very grave harm to the one who confided it, to the one who received it or to a third party, and where the very grave harm can be avoided only by divulging the truth. Even if not confided under the seal of secrecy, private information prejudicial to another is not to be divulged without a grave and proportionate reason.
2492 Everyone should observe an appropriate reserve concerning persons' private lives. Those in charge of communications should maintain a fair balance between the requirements of the common good and respect for individual rights. Interference by the media in the private lives of persons engaged in political or public activity is to be condemned to the extent that it infringes upon their privacy and freedom. 
V. THE USE OF THE SOCIAL COMMUNICATIONS MEDIA
2493 Within modern society the communications media play a major role in information, cultural promotion, and formation. This role is increasing, as a result of technological progress, the extent and diversity of the news transmitted, and the influence exercised on public opinion.
2494 The information provided by the media is at the service of the common good. [Cf. IM 11] Society has a right to information based on truth, freedom, justice, and solidarity: 
The proper exercise of this right demands that the content of the communication be true and - within the limits set by justice and charity - complete. Further, it should be communicated honestly and properly. This means that in the gathering and in the publication of news, the moral law and the legitimate rights and dignity of man should be upheld. [IM 5 § 2]
2495 "It is necessary that all members of society meet the demands of justice and charity in this domain. They should help, through the means of social communication, in the formation and diffusion of sound public opinion." [IM 8] Solidarity is a consequence of genuine and right communication and the free circulation of ideas that further knowledge and respect for others. 
2496 The means of social communication (especially the mass media) can give rise to a certain passivity among users, making them less than vigilant consumers of what is said or shown. Users should practice moderation and discipline in their approach to the mass media. They will want to form enlightened and correct consciences the more easily to resist unwholesome influences. 
2497 By the very nature of their profession, journalists have an obligation to serve the truth and not offend against charity in disseminating information. They should strive to respect, with equal care, the nature of the facts and the limits of critical judgment concerning individuals. They should not stoop to defamation.
2498 "Civil authorities have particular responsibilities in this field because of the common good.... It is for the civil authority ... to defend and safeguard a true and just freedom of information." [IM 12] By promulgating laws and overseeing their application, public authorities should ensure that "public morality and social progress are not gravely endangered" through misuse of the media. [IM 12 § 2] Civil authorities should punish any violation of the rights of individuals to their reputation and privacy. They should give timely and reliable reports concerning the general good or respond to the well-founded concerns of the people. Nothing can justify recourse to disinformation for manipulating public opinion through the media. Interventions by public authority should avoid injuring the freedom of individuals or groups. [2237, 2286]
2499 Moral judgment must condemn the plague of totalitarian states which systematically falsify the truth, exercise political control of opinion through the media, manipulate defendants and witnesses at public trials, and imagine that they secure their tyranny by strangling and repressing everything they consider "thought crimes." 
VI. TRUTH, BEAUTY, AND SACRED ART
2500 The practice of goodness is accompanied by spontaneous spiritual joy and moral beauty. Likewise, truth carries with it the joy and splendor of spiritual beauty. Truth is beautiful in itself. Truth in words, the rational expression of the knowledge of created and uncreated reality, is necessary to man, who is endowed with intellect. But truth can also find other complementary forms of human expression, above all when it is a matter of evoking what is beyond words: the depths of the human heart, the exaltations of the soul, the mystery of God. Even before revealing himself to man in words of truth, God reveals himself to him through the universal language of creation, the work of his Word, of his wisdom: the order and harmony of the cosmos-which both the child and the scientist discover-"from the greatness and beauty of created things comes a corresponding perception of their Creator," "for the author of beauty created them." [Wis 13:3, 5] [1804, 341, 2129]
[Wisdom] is a breath of the power of God, and a pure emanation of the glory of the Almighty; therefore nothing defiled gains entrance into her. For she is a reflection of eternal light, a spotless mirror of the working of God, and an image of his goodness. [Wis 7:25-26] For [wisdom] is more beautiful than the sun, and excels every constellation of the stars. Compared with the light she is found to be superior, for it is succeeded by the night, but against wisdom evil does not prevail. [Wis 7:29-30] I became enamored of her beauty. [Wis 8:2] 2501 Created "in the image of God," [Gen 1:26] man also expresses the truth of his relationship with God the Creator by the beauty of his artistic works. Indeed, art is a distinctively human form of expression; beyond the search for the necessities of life which is common to all living creatures, art is a freely given superabundance of the human being's inner riches. Arising from talent given by the Creator and from man's own effort, art is a form of practical wisdom, uniting knowledge and skill, [Cf. Wis 7:16-17] to give form to the truth of reality in a language accessible to sight or hearing. To the extent that it is inspired by truth and love of beings, art bears a certain likeness to God's activity in what he has created. Like any other human activity, art is not an absolute end in itself, but is ordered to and ennobled by the ultimate end of man. [Cf. Pius XII, Musicae sacrae disciplina; Discourses of September 3 and December 25, 1950] 
2502 Sacred art is true and beautiful when its form corresponds to its particular vocation: evoking and glorifying, in faith and adoration, the transcendent mystery of God - the surpassing invisible beauty of truth and love visible in Christ, who "reflects the glory of God and bears the very stamp of his nature," in whom "the whole fullness of deity dwells bodily." [Heb 1:3; Col 2:9] This spiritual beauty of God is reflected in the most holy Virgin Mother of God, the angels, and saints. Genuine sacred art draws man to adoration, to prayer, and to the love of God, Creator and Savior, the Holy One and Sanctifier. [1156-1162]
2503 For this reason bishops, personally or through delegates, should see to the promotion of sacred art, old and new, in all its forms and, with the same religious care, remove from the liturgy and from places of worship everything which is not in conformity with the truth of faith and the authentic beauty of sacred art. [Cf. SC 122-127]
2504 "You shall not bear false witness against your neighbor" (Ex 20:16).
Christ's disciples have "put on the new man, created after the likeness of God in true righteousness and holiness" (Eph 4:24).
2505 Truth or truthfulness is the virtue which consists in showing oneself true in deeds and truthful in words, and guarding against duplicity, dissimulation, and hypocrisy.
2506 The Christian is not to "be ashamed of testifying to our Lord" (2 Tim 1:8) in deed and word. Martyrdom is the supreme witness given to the truth of the faith.
2507 Respect for the reputation and honor of persons forbids all detraction and calumny in word or attitude.
2508 Lying consists in saying what is false with the intention of deceiving one's neighbor.
2509 An offense committed against the truth requires reparation.
2510 The golden rule helps one discern, in concrete situations, whether or not it would be appropriate to reveal the truth to someone who asks for it.
2511 "The sacramental seal is inviolable" (CIC, can. 983 § 1). Professional secrets must be kept. Confidences prejudicial to another are not to be divulged.
2512 Society has a right to information based on truth, freedom,
2513 The fine arts, but above all sacred art, "of their nature are directed toward expressing in some way the infinite beauty of God in works made by human hands. Their dedication to the increase of God's praise and of his glory is more complete, the more exclusively they are devoted to turning men's minds devoutly toward God" (SC 122).
THE NINTH COMMANDMENT
You shall not covet your neighbor's house; you shall not covet your neighbor's wife, or his manservant, or his maidservant, or his ox, or his ass, or anything that is your neighbor's. [Ex 20:17]
Every one who looks at a woman lustfully has already committed adultery with her in his heart. [Mt 5:28]
2514 St. John distinguishes three kinds of covetousness or concupiscence: lust of the flesh, lust of the eyes, and pride of life. [Cf. 1 Jn 2:16] In the Catholic catechetical tradition, the ninth commandment forbids carnal concupiscence; the tenth forbids coveting another's goods. [377, 400]
2515 Etymologically, "concupiscence" can refer to any intense form of human desire. Christian theology has given it a particular meaning: the movement of the sensitive appetite contrary to the operation of the human reason. The apostle St. Paul identifies it with the rebellion of the "flesh" against the "spirit." [Cf. Gal 5:16, 17, 24; Eph 2:3] Concupiscence stems from the disobedience of the first sin. It unsettles man's moral faculties and, without being in itself an offense, inclines man to commit sins. [Cf. Gen 3:11; Council of Trent: DS 1515] 
2516 Because man is a composite being, spirit and body, there already exists a certain tension in him; a certain struggle of tendencies between "spirit" and "flesh" develops. But in fact this struggle belongs to the heritage of sin. It is a consequence of sin and at the same time a confirmation of it. It is part of the daily experience of the spiritual battle: [362, 407]
For the Apostle it is not a matter of despising and condemning the body which with the spiritual soul constitutes man's nature and personal subjectivity. Rather, he is concerned with the morally good or bad works, or better, the permanent dispositions - virtues and vices - which are the fruit of submission (in the first case) or of resistance (in the second case) to the saving action of the Holy Spirit. For this reason the Apostle writes: "If we live by the Spirit, let us also walk by the Spirit." [John Paul II, DeV 55; cf. Gal 5:25]
I. PURIFICATION OF THE HEART
2517 The heart is the seat of moral personality: "Out of the heart come evil thoughts, murder, adultery, fornication...." [Mt 15:19] The struggle against carnal covetousness entails purifying the heart and practicing temperance: [368, 1809]
Remain simple and innocent, and you will be like little children who do not know the evil that destroys man's life. [Pastor Hermae, Mandate 2, 1: PG 2, 916]
2518 The sixth beatitude proclaims, "Blessed are the pure in heart, for they shall see God." [Mt 5:8] "Pure in heart" refers to those who have attuned their intellects and wills to the demands of God's holiness, chiefly in three areas: charity; [Cf. 1 Tim 4:3-9; 2 Tim 2:22] chastity or sexual rectitude; [Cf. 1 Thess 4:7; Col 3:5; Eph 4:19] love of truth and orthodoxy of faith. [Cf. Titus 1:15; 1 Tim 1:3-4; 2 Tim 2:23-26] There is a connection between purity of heart, of body, and of faith: 
The faithful must believe the articles of the Creed "so that by believing they may obey God, by obeying may live well, by living well may purify their hearts, and with pure hearts may understand what they believe." [St. Augustine, Defide et symbolo 10, 25: PL 40, 196] 
2519 The "pure in heart" are promised that they will see God face to face and be like him. [Cf. 1 Cor 13:12; 1 Jn 3:2] Purity of heart is the precondition of the vision of God. Even now it enables us to see according to God, to accept others as "neighbors"; it lets us perceive the human body - ours and our neighbor's - as a temple of the Holy Spirit, a manifestation of divine beauty. [2548, 2819, 2501]
II. THE BATTLE FOR PURITY
2520 Baptism confers on its recipient the grace of purification from all sins. But the baptized must continue to struggle against concupiscence of the flesh and disordered desires. With God's grace he will prevail 
- by the virtue and gift of chastity, for chastity lets us love with upright and undivided heart; 
- by purity of intention which consists in seeking the true end of man: with simplicity of vision, the baptized person seeks to find and to fulfill God's will in everything; [Cf. Rom 12:2; Col 1:10] 
- by purity of vision, external and internal; by discipline of feelings and imagination; by refusing all complicity in impure thoughts that incline us to turn aside from the path of God's commandments: "Appearance arouses yearning in fools"; [Wis 15:5] 
- by prayer: 
I thought that continence arose from one's own powers, which I did not recognize in myself. I was foolish enough not to know... that no one can be continent unless you grant it. For you would surely have granted it if my inner groaning had reached your ears and I with firm faith had cast my cares on you. [St. Augustine, Conf. 6, 11, 20: PL 32, 729-730]
2521 Purity requires modesty, an integral part of temperance. Modesty protects the intimate center of the person. It means refusing to unveil what should remain hidden. It is ordered to chastity to whose sensitivity it bears witness. It guides how one looks at others and behaves toward them in conformity with the dignity of persons and their solidarity.
2522 Modesty protects the mystery of persons and their love. It encourages patience and moderation in loving relationships; it requires that the conditions for the definitive giving and commitment of man and woman to one another be fulfilled. Modesty is decency. It inspires one's choice of clothing. It keeps silence or reserve where there is evident risk of unhealthy curiosity. It is discreet. 
2523 There is a modesty of the feelings as well as of the body. It protests, for example, against the voyeuristic explorations of the human body in certain advertisements, or against the solicitations of certain media that go too far in the exhibition of intimate things. Modesty inspires a way of life which makes it possible to resist the allurements of fashion and the pressures of prevailing ideologies. 
2524 The forms taken by modesty vary from one culture to another. Everywhere, however, modesty exists as an intuition of the spiritual dignity proper to man. It is born with the awakening consciousness of being a subject. Teaching modesty to children and adolescents means awakening in them respect for the human person.
2525 Christian purity requires a purification of the social climate. It requires of the communications media that their presentations show concern for respect and restraint. Purity of heart brings freedom from widespread eroticism and avoids entertainment inclined to voyeurism and illusion. 
2526 So called moral permissiveness rests on an erroneous conception of human freedom; the necessary precondition for the development of true freedom is to let oneself be educated in the moral law. Those in charge of education can reasonably be expected to give young people instruction respectful of the truth, the qualities of the heart, and the moral and spiritual dignity of man. 
2527 "The Good News of Christ continually renews the life and culture of fallen man; it combats and removes the error and evil which flow from the ever-present attraction of sin. It never ceases to purify and elevate the morality of peoples. It takes the spiritual qualities and endowments of every age and nation, and with supernatural riches it causes them to blossom, as it were, from within; it fortifies, completes, and restores them in Christ." [GS 58 § 4] 
2528 "Everyone who looks at a woman lustfully has already committed adultery with her in his heart" (Mt 5:28).
2529 The ninth commandment warns against lust or carnal concupiscence.
2530 The struggle against carnal lust involves purifying the heart and practicing temperance.
2531 Purity of heart will enable us to see God: it enables us even now to see things according to God.
2532 Purification of the heart demands prayer, the practice of chastity, purity of intention and of vision.
2533 Purity of heart requires the modesty which is patience, decency, and discretion. Modesty protects the intimate center of the person.
THE TENTH COMMANDMENT
You shall not covet... anything that is your neighbor's.... You shall not desire your neighbor's house, his field, or his manservant, or his maidservant,, or his ox, or his ass, or anything that is your neighbor's. [EX 20:17; Deut 5:21]
For where your treasure is, there will your heart be also. [Mt 6:21]
2534 The tenth commandment unfolds and completes the ninth, which is concerned with concupiscence of the flesh. It forbids coveting the goods of another, as the root of theft, robbery, and fraud, which the seventh commandment forbids. "Lust of the eyes" leads to the violence and injustice forbidden by the fifth commandment. [Cf. 1 Jn 2:16; Mic 2:2] Avarice, like fornication, originates in the idolatry prohibited by the first three prescriptions of the Law. [Cf. Wis 14:12] The tenth commandment concerns the intentions of the heart; with the ninth, it summarizes all the precepts of the Law. [2112, 2069]
I. THE DISORDER OF COVETOUS DESIRES
2535 The sensitive appetite leads us to desire pleasant things we do not have, e.g., the desire to eat when we are hungry or to warm ourselves when we are cold. These desires are good in themselves; but often they exceed the limits of reason and drive us to covet unjustly what is not ours and belongs to another or is owed to him. 
2536 The tenth commandment forbids greed and the desire to amass earthly goods without limit. It forbids avarice arising from a passion for riches and their attendant power. It also forbids the desire to commit injustice by harming our neighbor in his temporal goods: 
When the Law says, "You shall not covet," these words mean that we should banish our desires for whatever does not belong to us. Our thirst for another's goods is immense, infinite, never quenched. Thus it is written: "He who loves money never has money enough." [Roman Catechism, III, 37; cf. Sir 5:8]
2537 It is not a violation of this commandment to desire to obtain things that belong to one's neighbor, provided this is done by just means. Traditional catechesis realistically mentions "those who have a harder struggle against their criminal desires" and so who "must be urged the more to keep this commandment":
...merchants who desire scarcity and rising prices, who cannot bear not to be the only ones buying and selling so that they themselves can sell more dearly and buy more cheaply; those who hope that their peers will be impoverished, in order to realize a profit either by selling to them or buying from them... physicians who wish disease to spread; lawyers who are eager for many important cases and trials. [Roman Catechism, III, 37]
2538 The tenth commandment requires that envy be banished from the human heart. When the prophet Nathan wanted to spur King David to repentance, he told him the story about the poor man who had only one ewe lamb that he treated like his own daughter and the rich man who, despite the great number of his flocks, envied the poor man and ended by stealing his lamb. [Cf. 2 Sam 12:14] Envy can lead to the worst crimes. [Cf. Gen 4:3-7; 1 Kings 21:1-29] "Through the devil's envy death entered the world": [Wis 2:24] [2317, 391]
We fight one another, and envy arms us against one another.... If everyone strives to unsettle the Body of Christ, where shall we end up? We are engaged in making Christ's Body a corpse.... We declare ourselves members of one and the same organism, yet we devour one another like beasts. [St. John Chrysostom, Hom. in 2 Cor. 27, 3-4 PG 61, 588]
2539 Envy is a capital sin. It refers to the sadness at the sight of another's goods and the immoderate desire to acquire them for oneself, even unjustly. When it wishes grave harm to a neighbor it is a mortal sin: 
St. Augustine saw envy as "the diabolical sin." [Cf. St. Augustine, De catechizandis rudibus 4, 8 PL 40, 315-316] "From envy are born hatred, detraction, calumny, joy caused by the misfortune of a neighbor, and displeasure caused by his prosperity." [St. Gregory the Great Moralia in Job 31, 45: PL 76, 621]
2540 Envy represents a form of sadness and therefore a refusal of charity; the baptized person should struggle against it by exercising good will. Envy often comes from pride; the baptized person should train himself to live in humility: 
Would you like to see God glorified by you? Then rejoice in your brother's progress and you will immediately give glory to God. Because his servant could conquer envy by rejoicing in the merits of others, God will be praised. [St. John Chrysostom, Hom. in Rom. 71, 5: PG 60, 448]
II. THE DESIRES OF THE SPIRIT
2541 The economy of law and grace turns men's hearts away from avarice and envy. It initiates them into desire for the Sovereign Good; it instructs them in the desires of the Holy Spirit who satisfies man's heart. [1718, 2764]
The God of the promises always warned man against seduction by what from the beginning has seemed "good for food... a delight to the eyes... to be desired to make one wise." [Gen 3:6] 
2542 The Law entrusted to Israel never sufficed to justify those subject to it; it even became the instrument of "lust." [Cf. Rom 7:7] The gap between wanting and doing points to the conflict between God's Law which is the "law of my mind," and another law "making me captive to the law of sin which dwells in my members." [Rom 7:23; cf. 7:10] 
1543 "But now the righteousness of God has been manifested apart from law, although the law and the prophets bear witness to it, the righteousness of God through faith in Jesus Christ for all who believe." [Rom 3:21-22] Henceforth, Christ's faithful "have crucified the flesh with its passions and desires"; they are led by the Spirit and follow the desires of the Spirit. [Gal 5:24; cf. Rom 8:14, 27] 
III. POVERTY OF HEART [2443-2449]
2544 Jesus enjoins his disciples to prefer him to everything and everyone, and bids them "renounce all that [they have]" for his sake and that of the Gospel. [Lk 14:33; cf. Mk 8:35] Shortly before his passion he gave them the example of the poor widow of Jerusalem who, out of her poverty, gave all that she had to live on. [Cf. Lk 21:4] The precept of detachment from riches is obligatory for entrance into the Kingdom of heaven. 
2545 All Christ's faithful are to "direct their affections rightly, lest they be hindered in their pursuit of perfect charity by the use of worldly things and by an adherence to riches which is contrary to the spirit of evangelical poverty." [LG 42 § 3] 
2546 "Blessed are the poor in spirit." [Mt 5:3] The Beatitudes reveal an order of happiness and grace, of beauty and peace. Jesus celebrates the joy of the poor, to whom the Kingdom already belongs: [Cf. Lk 6:20] 
The Word speaks of voluntary humility as "poverty in spirit"; the Apostle gives an example of God's poverty when he says: "For your sakes he became poor." [St. Gregory of Nyssa, De beatitudinibus 1: PG 44, 1200D; cf. 2 Cor 8:9]
2547 The Lord grieves over the rich, because they find their consolation in the abundance of goods. [Lk 6:24] "Let the proud seek and love earthly kingdoms, but blessed are the poor in spirit for theirs is the Kingdom of heaven." [St. Augustine, De serm. Dom. in monte 1, 1, 3: PL 34, 1232] Abandonment to the providence of the Father in heaven frees us from anxiety about tomorrow. [Cf. Mt 6:25-34] Trust in God is a preparation for the blessedness of the poor. They shall see God. 
IV. "I WANT TO SEE GOD"
2548 Desire for true happiness frees man from his immoderate attachment to the goods of this world so that he can find his fulfillment in the vision and beatitude of God. "The promise [of seeing God] surpasses all beatitude.... In Scripture, to see is to possess.... Whoever sees God has obtained all the goods of which he can conceive." [St. Gregory of Nyssa, De beatitudinibus 6: PG 44, 1265A] 
2549 It remains for the holy people to struggle, with grace from on high, to obtain the good things God promises. In order to possess and contemplate God, Christ's faithful mortify their cravings and, with the grace of God, prevail over the seductions of pleasure and power. 
2550 On this way of perfection, the Spirit and the Bride call whoever hears them [Cf. Rev 22:17] to perfect communion with God:
There will true glory be, where no one will be praised by mistake or flattery; true honor will not be refused to the worthy, nor granted to the unworthy; likewise, no one unworthy will pretend to be worthy, where only those who are worthy will be admitted. There true peace will reign, where no one will experience opposition either from self or others. God himself will be virtue's reward; he gives virtue and has promised to give himself as the best and greatest reward that could exist.... "I shall be their God and they will be my people...." This is also the meaning of the Apostle's words: "So that God may be all in all." God himself will be the goal of our desires; we shall contemplate him without end, love him without surfeit, praise him without weariness. This gift, this state, this act, like eternal life itself, will assuredly be common to all. [St. Augustine, De civ. Dei, 22, 30: PL 41, 801-802; cf. Lev 26:12; cf. 1 Cor 15:28] 
2551 "Where your treasure is, there will your heart be also" (Mt 6:21).
2552 The tenth commandment forbids avarice arising from a passion for riches and their attendant power.
2553 Envy is sadness at the sight of another's goods and the immoderate desire to have them for oneself. It is a capital sin.
2554 The baptized person combats envy through good-will, humility, and abandonment to the providence of God.
2555 Christ's faithful "have crucified the flesh with its passions and desires" (Gal 5:24); they are led by the Spirit and follow his desires.
2556 Detachment from riches is necessary for entering the Kingdom of heaven. "Blessed are the poor in spirit."
2557 "I want to see God" expresses the true desire of man. Thirst for God is quenched by the water of eternal life (cf. In 4:14).