The Leipzig Debate in the summer of 1519 proved significant in pushing the indulgence controversy beyond the question of penance and justification to the question of authority in the church. John Eck, a scholastic theologian teaching at Ingolstadt, had engaged Luther in private correspondence on the issue of indulgences, but that correspondence was published against Luther’s own wishes. In defense of his Wittenberg colleague, Andreas Bodenstein von Karlstadt wrote an extensive repudiation of Eck’s treatise, including well over 300 theses addressing questions of grace, free will, and penance. Eck replied in kind with another set of theses. The written controversy led to demand for a debate, initially restricted to Eck and Karlstadt, but later including Luther. It was to be held at Leipzig under the patronage of Duke George of Saxony.
The literature preceding the debate had raised the question of whether Luther rejected papal authority in the same way as the Bohemian heretic Jan Hus. Debating the nature of papal authority was proscribed, but Duke George allowed it to go forward. Prior to the debate Luther privately admitted to his friend Georg Spalatin a rising belief that the pope was of some relation to the antichrist—echoing claims made by others before him in the Middle Ages—but publicly he continued to affirm the papacy as established by God for the unity of the church. Then in a sermon on the eve of the debate in Leipzig, he emphasized the centrality of faith in Christ as the basis for salvation, not papal fiat. But at the outset of the debate, all three participants were required to announce publicly their commitment to orthodoxy and Luther did so with the qualification that he was only debating papal authority because Eck had pressured him to do so, not of his own volition.
After the early stages of the debate involved an exchange between Karlstadt and Eck on grace and free will, Luther and Eck then shifted the discussion toward authority in the church. While Eck defended papal authority from traditional proof texts, such as Matthew 16, Luther rejected them on the grounds of exegesis. He accepted the papacy as a divine institution, but he did not accept Eck’s interpretation of those texts as attributing authority over salvific matters to the papacy. In the course of the debate, Luther eventually responded to the claim that he was supporting a condemned position by defending many of Hus’s positions as essentially orthodox. When Eck questioned his defense of a noted heretic, Luther countered that the Council of Constance, which sentenced Hus to death, could have been in error. This led him to state that councils could and had erred, as had popes and canon law. What remained infallible for Luther was Scripture and thus it was finally authoritative for the church.
For the first time, Luther had articulated clearly his position that popes, councils, and theologians were all subject to error, leaving Scripture as the supreme authority in all theological matters. This became a watershed moment, resulting in both increased support and increased opposition after he left Leipzig. It even led to correspondence with the Bohemians, after which Luther famously exclaimed in agreement with their doctrine of the church: “We are all Hussites.” It is worth noting, however, that after Leipzig Luther continued to support the papacy as a divine institution, though he disagreed with the extent of its authority.