Originally Posted by Catholic.com
As from the first, God speaks to his Church through the Bible and through sacred Tradition. To make sure we understand him, he guides the Church’s teaching authority—the magisterium—so it always interprets the Bible and Tradition accurately. This is the gift of infallibility.
Like the three legs on a stool, the Bible, Tradition, and the magisterium are all necessary for the stability of the Church and to guarantee sound doctrine.
Sacred Tradition (CCC 75–83)
Sacred Tradition should not be confused with mere traditions of men, which are more commonly called customs or disciplines. Jesus sometimes condemned customs or disciplines, but only if they were contrary to God’s commands (Mark 7:8). He never condemned sacred Tradition, and he didn’t even condemn all human tradition.
Sacred Tradition and the Bible are not different or competing revelations. They are two ways that the Church hands on the gospel. Apostolic teachings such as the Trinity, infant baptism, the inerrancy of the Bible, purgatory, and Mary’s perpetual virginity have been most clearly taught through Tradition, although they are also implicitly present in (and not contrary to) the Bible. The Bible itself tells us to hold fast to Tradition, whether it comes to us in written or oral form (2 Thess. 2:15, 1 Cor. 11:2).
Sacred Tradition should not be confused with customs and disciplines, such as the rosary, priestly celibacy, and not eating meat on Fridays in Lent. These are good and helpful things, but they are not doctrines. Sacred Tradition preserves doctrines first taught by Jesus to the apostles and later passed down to us through the apostles’ successors, the bishops.
Scripture (CCC 101–141)
Scripture, by which we mean the Old and New Testaments, was inspired by God (2 Tim. 3:16). The Holy Spirit guided the biblical authors to write what he wanted them to write. Since God is the principal author of the Bible, and since God is truth itself (John 14:6) and cannot teach anything untrue, the Bible is free from all error in everything it asserts to be true.
Some Christians claim, "The Bible is all I need," but this notion is not taught in the Bible itself. In fact, the Bible teaches the contrary idea (2 Pet. 1:20–21, 3:15–16). The "Bible alone" theory was not believed by anyone in the early Church.
It is new, having arisen only in the 1500s during the Protestant Reformation. The theory is a "tradition of men" that nullifies the Word of God, distorts the true role of the Bible, and undermines the authority of the Church Jesus established (Mark 7:1–8).
Although popular with many "Bible Christian" churches, the "Bible alone" theory simply does not work in practice. Historical experience disproves it. Each year we see additional splintering among "Bible-believing" religions.
Today there are tens of thousands of competing denominations, each insisting its interpretation of the Bible is the correct one. The resulting divisions have caused untold confusion among millions of sincere but misled Christians.
Just open up the Yellow Pages of your telephone book and see how many different denominations are listed, each claiming to go by the "Bible alone," but no two of them agreeing on exactly what the Bible means.
We know this for sure: The Holy Spirit cannot be the author of this confusion (1 Cor. 14:33). God cannot lead people to contradictory beliefs because his truth is one. The conclusion? The "Bible alone" theory must be false.
The Magisterium (CCC 85–87, 888–892)
Together the pope and the bishops form the teaching authority of the Church, which is called the magisterium (from the Latin for "teacher"). The magisterium, guided and protected from error by the Holy Spirit, gives us certainty in matters of doctrine. The Church is the custodian of the Bible and faithfully and accurately proclaims its message, a task which God has empowered it to do.
Keep in mind that the Church came before the New Testament, not the New Testament before the Church. Divinely-inspired members of the Church wrote the books of the New Testament, just as divinely-inspired writers had written the Old Testament, and the Church is guided by the Holy Spirit to guard and interpret the entire Bible, both Old and New Testaments.
Such an official interpreter is absolutely necessary if we are to understand the Bible properly. (We all know what the Constitution says, but we still need a Supreme Court to interpret what it means.)
The magisterium is infallible when it teaches officially because Jesus promised to send the Holy Spirit to guide the apostles and their successors "into all truth" (John 16:12–13).