The Church's faith is that God has revealed Himself to mankind, providing a secure way of knowing Him, and His plan for the human race.
This Divine Revelation was first given to the Israelites through Moses, then through the prophets, as well as the other inspired writers, and is contained in what Jews call the Tanach and Christians the Old Testament.
The Revelation to Israel, and the Covenant associated with it, both prefigured and prepared the way for an even greater Revelation and Covenant, that of Jesus Christ. The Lord committed this Revelation, Old and New, to the Apostles, who handed it on in the Church, either in written form or through their preaching and teaching.
The written testimony of the sacred writers of the Old and New Covenant constitute Sacred Scripture. The preaching and teaching of the apostles constitute Sacred Tradition, the Apostolic Faith "handed on" (Latin, "traditio") in the Church (1 Cor. 11:2; 2 Thess. 2:15, 3:6). It is attested to by the essential elements of the liturgical/sacramental life of the Church, and in the writings of the early bishops and other teachers of the faith (the Fathers of the Church) when they agreed that a teaching is of apostolic origin.
Such Sacred, or Apostolic, Traditions originating with Christ and the Apostles are distinguished from human traditions, which Christ (Mk 7:1-8) and St. Paul (Col. 2:8) condemned when they conflicted with Revelation. Sacred Traditions are different because Christ and the Apostles speak with authority (Mt. 7:29, 16: 17; 18:17-18; Lk. 10:16) – Christ on behalf of His Father, the Apostles on behalf of Christ. All other human beings speak only for themselves, and their teaching and traditions rest solely on reason and the logical coherence of their arguments.
Sacred Scripture and Sacred Tradition taken together, therefore, are two sources of God's one Revelation of Himself, fulfilled in Jesus Christ. This being so, with the death of the last witness to Christ's life, death and resurrection (St. John the Apostle, c. 98 AD), the period of Public Revelation closed. Since then, nothing can be added or subtracted from Divine Revelation, and the Church rejects claims to do so, whether by Islam, Mormonism, the New Age movement or others.
Christ continues to guarantee the authenticity of His Revelation, however. Before returning to His Father He committed His Divine mission to the Apostles (Mt. 28:18-20), and they, before leaving this world, handed it on to the bishops whom they appointed to succeed them as shepherds of the Church (Acts 20:28ff; 1 & 2 Tim). This ensures that salvation is preached to the whole world in all centuries, until the Lord returns.
Unlike the Apostles, the bishops are not immediate witnesses to Christ, and thus do not share in that aspect of the apostolic office. Bishops cannot directly contribute, therefore, to Divine Revelation. Like the Apostles, however, they are pastors of Christ's flock, and they continue in that office until the Good Shepherd returns at the end of time. Their mission is to guard and protect what the Lord handed on to them through the Apostles, ensuring both that the Church faithfully teaches what is already held as "of the faith," and that any new insights into Divine Revelation (theological development) are consistent with it.
For this the bishops of Rome who succeed Peter in his office are guaranteed the grace to be able to "strengthen the brethren in the faith" (Luke 22:31-32), while the successors of the apostles collectively are promised the Holy Spirit to lead them to all truth (John 16:13). Individual bishops, or even entire national hierarchies, can fail to adhere to the truth, but the Pope and the Bishops as a whole cannot.
History shows this to be true in all matters taught as revealed by God. At the time of the Arian heresy the Italian bishops followed Arius, save for the Pope and St. Ambrose in Milan. In Tudor England, the English hierarchy all capitulated to Henry VIII on his claim to supremacy over the Church in England, save St. John Fischer. This has been repeated again and again as the Church survives the defection of her faithful and even some of her pastors.
Twenty centuries after Christ, however, the Church continues to teach the same faith, the same morality, as found in the New Testament, the Didache (c. 100 AD), the Fathers of the Church, and the popes, bishops and theologians of the last two millennia. This includes on abortion, birth control and human sexuality.