Q. 1. What is the meaning of "Deposit of Faith?"
A. 1. The Catholic Church defines the "Deposit of Faith" as the teachings of the Catholic Church that are handed down since the time of the Apostles through Sacred Scripture and Sacred Tradition. These two are known as the "Deposit of Faith." (depositum fidei).
The Sacred Scripture and Sacred Tradition (deposit of faith) are in turn interpreted by the Magisterium (Latin for "teacher"), the Church's teaching authority, which is exercised by the Pope and the College of Bishops in union with the Pope who is the bishop of Rome.
The Catholic belief regarding the deposit of faith is that Christ left behind all the knowledge that was needed (Public Revelation), in both textual (Scripture) and verbal instructions (Tradition). The verbal instructions were teachings but the correct interpretation of Scriptures and the Words of Christ, being instilled in the Apostles who then instilled these teachings regarding Scriptures, etc... through Apostolic Succession to those they ordained to take their places. While this Tradition is not recorded in Scripture itself, it is recorded in the teachings of the Fathers of the Church in the early years. For example, it is recorded in the writings of Polycarp and his disciple Ignatius, it is echoed through the teachings of the church down the ages.
In summary, the "Deposit of Faith" is the body of revealed truth in the Scriptures and Tradition taught by the Roman Catholic Church for the belief of the faithful.
It is the mission of the Church, which the Lord has entrusted to the Catholic Church, to guard the "Deposit of Faith." Since the "Deposit of Faith" cannot change, it is the responsibility of the Church to preserve, protect, and interpret it. Pope John Paul II stated in the introduction of this catechism that “Guarding the deposit of faith is the mission which the Lord entrusted to His Church, and she fulfills in every age.” As Catholics, we believe that Christ established an authoritative Church, through the Magisterium or teaching authority, that has the ability to define the truth of Christ’s teachings.
That is why, we as Catholics, are required, without exception, to believe in all of the teachings of the Catholic Church. We cannot say, "I believe this, but I do not believe that."