The Catholic Church bases her teaching upon one source: The word of God. This divine revelation is transmitted in two ways: through Scripture and apostolic tradition. Many assume that only the writings of the apostles are the word of God. However, their oral transmission of the faith is also considered the word of God (1 Thess. 2:13). Few Protestant groups today accept the validity, let alone the authority, of tradition. In fact, many believe that Scripture is the only definitive source of divine truth.
Protestants claim the Bible is the only rule of faith, meaning that it contains all of the material one needs for theology and that this material is sufficiently clear that one does not need apostolic tradition or the Church’s magisterium (teaching authority) to help one understand it. In the Protestant view, the whole of Christian truth is found within the Bible’s pages. Anything extraneous to the Bible is simply non-authoritative, unnecessary, or wrong—and may well hinder one in coming to God.
~ Rene Laennec; French physician, father of modern pulmonary disease research; to his wife, after he was thrown from their carriage in an accident; whereupon they went on with the rosary they had been reciting just before the accident.