Sola Scriptura began when Martin Luther decided to deny the infallibility of the Church, ecumenical councils and the pope — i.e., claiming that they could err — thus leaving the Bible as the only infallible rule of faith.

The novelty wasn’t mentioned in Luther’s 95 Theses (posted Oct. 31, 1517). But Protestant Luther biographer Roland Bainton (Here I Stand) stated that in 1518 “Luther ... had further declared the pope and councils to be capable of error.” Then he reiterated and strengthened such claims during his 18-day Leipzig Disputation of July 1519, with Johann Eck. Bainton reports that Luther stated:

I assert that a council has sometimes erred and may sometimes err. Nor has a council authority to establish new articles of faith. A council cannot make divine right out of that which by nature is not divine right. Councils have contradicted each other, for the recent Lateran Council has reversed the claim of the councils of Constance and Basel that a council is above a pope. A simple layman armed with Scripture is to be believed above a pope or a council without it. ...  I say that neither the Church nor the pope can establish articles of faith. These must come from Scripture. For the sake of Scripture, we should reject pope and councils.

In 1520, Luther expressly denied papal infallibility: “They play about with words before our very eyes, trying to persuade us that the pope cannot err in matters of faith, regardless of whether he is righteous or wicked.” Even more radically, he wrote, “If we are all priests ... why should we not also have the power to test and judge what is right or wrong in matters of faith?” And, “Popes, bishops, canons, and monks. God has not instituted these offices.” Accordingly, he wrote in March 1521:

This is my answer to those also who accuse me of rejecting all the holy teachers of the church. I do not reject them. But everyone, indeed, knows that at times they have erred, as men will; therefore, I am ready to trust them only when they give me evidence for their opinions from Scripture, which has never erred.

The next big milestone in this tragic and utterly anti-traditional trajectory is the Diet of Worms (January to May 1521), where Luther made his famous and climactic statement on April 18:

Unless I am convinced by the testimony of the Scriptures or by clear reason (for I do not trust either in the pope or in councils alone, since it is well known that they have often erred and contradicted themselves), I am bound by the Scriptures I have quoted and my conscience is captive to the Word of God. I cannot and will not recant anything, since it is neither safe nor right to go against conscience. May God help me. Amen.

This is a rejection of the traditional rule of faith, which was taught in Holy Scripture, held by the Church Fathers, and by the entire Church up until Luther decided that he knew better than all of previous Catholic Church history. By rejecting papal, conciliar and ecclesiastical infallibility and saying that popes and ecumenical councils and the Church could err, Luther set up the standard definition of sola Scriptura: Nothing is infallible except Scripture, and it alone functions as the rule of faith in Christianity (formal sufficiency).

Critics of the Catholic Church often contend that the magisterium of the Catholic Church had not claimed that the pope was infallible until the 19th century. This is untrue. For example, Pope Clement VI had written in 1351:

[W]hen doubts arise concerning the catholic faith, the Roman pontiff alone is able to put an end [to them] by an authentic decision that must be adhered to inviolably; and what he himself determines to be true, by virtue of the keys handed over to him by Christ, is true and catholic, and what he determines to be false and heretical must be considered as such.

And a 1690 Decree of the Holy Office during the reign of Pope Alexander VIII (1689-1691), entitled Errors of the Jansenists, condemned the following error:

The assertion of the authority of the Roman Pontiff over an ecumenical council and infallibility in deciding questions of faith is futile and often contradicted.

The 16th-century Protestant confessions and standard works of theology followed Luther’s thinking. John Calvin disputed the Catholic contention that ecumenical councils were “under the immediate guidance of the Holy Spirit, and therefore cannot err” (Institutes of the Christian Religion, Bk, IV, 8:10) and thought the Roman See, “once the mother of all the churches,” exhibited in his day “nothing but horrible apostasy” and had become “the seat of Antichrist” (Ibid., IV, 7:34).

The Belgic Confession (1561) referred to the Bible as the sole “infallible rule.” The 2nd Helvetic Confession (1564) opined that “we do not permit ourselves, in controversies about religion or matters of faith, to urge our case with only the opinions of the fathers or decrees of councils ... we reject human traditions, even if they be adorned with high-sounding titles, as though they were divine and apostolical, delivered to the Church by the living voice of the apostles.”

The Anglican 39 Articles (1571) stated that “the Church of Rome hath erred ... in matters of faith” (Art. XIX) and that ecumenical councils “may err and sometime have erred, even in things pertaining to God” (Art. XXI).