N. 39. "Although these (Eastern Churches are separated from us, yet they possess true sacraments, above all - by apostolic succession - the priesthood and the Eucharist, whereby they are still joined to us in closest intimacy. Therefore some sharing in liturgical worship (communicato in sacris), given suitable circumstances and the approval of church authority, is not merely possible but is encouraged."
N. 41 The principles governing this sharing set out in the Decree on Eastern Churches should be observed with the prudence that the decree recommends; the norms which apply to Oriental Catholics apply equally to the faithful of any rite, including the Latin.
N. 42 It is particularly opportune that the Catholic authority, whether the local one, the synod or the episcopal conference, does not extend permission for sharing in the reception or administration of the sacraments of Penance, Holy Eucharist or Anointing of the Sick except after satisfactory consultations with the competent authorities (at least local ones) of the separated Oriental Church.
N. 47 A Catholic who occasionally, for reasons set out below, attends the Holy Liturgy (Mass) on a Sunday or holy day of obligation in an Orthodox Church is not then bound to assist at Mass in a Catholic Church. It is likewise a good thing if on such days Catholics who for just reasons cannot go to Mass in their own Church, attend the Holy Liturgy of their separated Oriental brethren, if this is possible.
N. 48 Because of the close communion between the Catholic Church and the separated Eastern Churches, as described above (n. 40), it is permissible for a member of one of the latter to act as a godparent, together with a Catholic godparent, at the baptism of a Catholic infant or adult so long as there is a provision for the Catholic education of the person being baptized, and it is clear that the godparent is a suitable one. A Catholic is not forbidden to stand as godparent in an Orthodox church, if he is invited. In this case, the duty of providing for the Christian education of the baptized person binds in the first place the godparent who belongs to the Church in which the child is baptized.
No. 49 Brethren of other churches may act as bridesmaid or best man at a wedding in a Catholic church. A Catholic too can be best man or bridesmaid at a marriage properly celebrated among separated brethren.
No. 50 Catholics may be allowed to attend Orthodox liturgical services if they have reasonable grounds, e.g., arising out of a public office or function, blood relationships, friendships, desire to be better informed, etc. In such cases there is nothing against their taking part in the common responses, hymns, and actions of the Church in which they are guests. Receiving Holy Communion, however, will be governed by what is laid down above, nn. 42 and 44. Because of the close communion referred to earlier (n. 40), local Ordinaries can give permission for a Catholic or read lessons at a liturgical service, if he is invited. These same principles govern the matter in which an Orthodox may assist at services in Catholic churches.
No. 55 Celebration of the sacraments is an action of the celebrating community, carried out within the community, signifying the oneness of faith, worship and life of the community. Where this unity of sacramental faith is deficient, the participation of the separated brethren with Catholics, especially in the sacraments of the Eucharist, Penance and Anointing of the Sick, is forbidden. Nevertheless, since the sacraments are both signs of unity and sources of grace, the Church can for adequate reasons allow access to those sacraments to a separated brother. This may be permitted in danger of death or in urgent need (during persecution, in prisons) if the separated brother has no access to a minister of his own communion, and spontaneously asks a Catholic priest for the sacraments - so long as he declares a faith in these sacraments in harmony with that of the Church, and is rightly disposed. In other cases the judge of this urgent necessity must be the diocesan bishop or the episcopal conference. A Catholic in similar circumstances may not ask for these sacraments except from a minister who has been validly ordained.
No. 56 A separated brother is not to act as a Scripture reader or to preach during the celebration of the Eucharist. The same is to be said of a Catholic at the celebration of the Lord's Supper or at the principal liturgical service of the Word held by the Christians who are separated from us. At other services, even liturgical ones, it is allowable to exercise some functions, with the previous permission of the local Ordinary and the consent of the authorities of the community concerned.
No. 57 With the exception already dealt with above (n. 48) it is not permissible for a member of a separated community to act as godparent in the liturgical and canonical sense at Baptism or Confirmation. The reason is that a godparent is not merely undertaking his responsibility for the Christian education of the person baptized or confirmed as a relation or friend - he is also, as a representative of a community of faith, standing as a sponsor for the faith of the candidate. Equally, a Catholic cannot fulfill this function for a member of a separated community. However, because of ties of blood of friendship, a Christian of another communion, since he has faith in Christ, can be admitted with a Catholic godparent as a Christian witness of the baptism. In comparable circumstances a Catholic can do the same for a member of a separated community. In these cases the responsibility for the Christian education of the candidate belongs of itself to the godparent who is a member of the Church in which the candidate is baptized. Pastors should explain carefully to the faithful the evangelical and ecumenical reasons for this regulation, so that all misunderstanding of it may be prevented.
No. 58 The separated brethren may act as "official" witnesses (bridesmaid or best man) at a Catholic marriage, and Catholics at a marriage which is properly celebrated between our separated brethren.
No. 59 Catholics may be allowed to attend occasionally the liturgical services of other brethren if they have reasonable ground, e.g., arising out of a public office or function, blood relationship or friendship, desire to be better informed, an ecumenical gathering, etc. In these cases, with due regard to what has been said above - there is nothing against Catholics taking some part in the common responses, hymns and actions of the community of which they are guests - so long as they are not at variance with Catholic faith. The same principles govern the manner in which our separated brethren may assist at services in Catholic churches. This participation, from which reception of the Eucharist is always excluded, should lead the participants to esteem the spiritual riches we have in common and at the same time make them more aware of the gravity of our separations.