The main principles of the Reformation, we have noted in previous articles, are usually considered to be two: the formal and the material. According to the formal principle, the Reformation acknowledged only one source of authority, the Holy Scriptures. With this principle they stood opposed to Roman Catholicism, False Mysticism (Quakers, Anabaptists, etc.), and to Rationalism. The Roman Catholic Church acknowledged, besides the Word of God, also Tradition as a source of authority; Protestantism recognized only the sixty-six canonical books.