The doctrine that the Bible alone is the ultimate authority was the “formal principle” of the Reformation. In 1521 at the historic interrogation at the Diet of Worms, Luther declared his conscience to be captive to the Word of God saying, “Unless I am overcome with testimonies from Scripture or with evident reasons—for I believe neither the Pope nor the Councils, since they have often erred and contradicted one another—I am overcome by the Scripture texts which I have adduced, and my conscience is bound by God’s Word.”
Similarly, the Belgic Confession states,
We believe that [the] holy Scriptures fully contain the will of God, and that whatsoever man ought to believe unto salvation is sufficiently taught therein. . . . Neither may we consider any writings of men, however holy these men may have been, of equal value with those divine Scriptures nor ought we to consider custom or the great multitude, or antiquity, or succession of times and persons, or councils, decrees or statutes, as of equal value with the truth of God. . . . Therefore, we reject with all our hearts whatsoever does not agree with this infallible rule.
When the Reformers used the words sola Scriptura they were expressing their concern for the Bible’s authority, and what they meant is that the Bible alone is our ultimate authority—not the Pope, not the church, not the traditions of the church or church councils, still less personal intimations or subjective feelings, but Scripture only. Other sources of authority may have an important role to play. Some are even established by God, such as the authority of church elders, the authority of the state, or the authority of parents over children. But Scripture alone is truly ultimate. Therefore, if any of these other authorities depart from Bible teaching, they are to be judged by the Bible and rejected.