For various political reasons, after Martin Luther's interview with Cardinal Cajetan
, Pope Leo X sought a more conciliatory relationship with Luther. Leo X sent an emissary, Carl von Miltitz, to Frederick the Wise. Von Miltitz was a cousin of Frederick. Miltitz spoke with Luther, and secured Luther's promise that he would no longer engage in debate and publication regarding indulgences, if those within the Catholic church who had been opposing Luther would likewise refrain from attacking his position.
This "truce" was short-lived, however, due to the actions of John Eck. Eck, a professor at the University of Ingolstadt, had already engaged in public debate with Luther through a publication titled Obelisks, which was intended to refute the 95 Theses, and he persuaded Duke George the Bearded of Saxony to sponsor a debate between himself and Andreas Carlstadt-- a Wittenberg professor who had become convinced of Luther's teachings-- at the University of Leipzig. Luther desired to defend his own teachings, and so he arranged to debate Eck at Leipzig as well.
In July 1519, after Eck and Carlstadt debated for a week concerning the depravity of man [showing, incidentally, the centrality of this doctrine to Protestant Reformational teaching], Luther debated Eck on the question of whether the papacy was of divine or human institution. According to the terms of the debate, neither Luther nor Eck could bring books to the table, but they were dependent upon what they had memorized. The proof quoted by Eck consisted of canon law and selections from church history. Though Luther had studied vigorously for the debate, he was no match for Eck in regards to these types of documents. Luther instinctively turned to the writings that he had memorized for years as a monk and as a professor of theology; in response to Eck, Luther quoted the Bible.
Luther's quoting the Bible against canon law brought the charge from Eck that Luther was a Hussite, since Jan Hus, who had been burned as a heretic a century before, had similarly argued against the papacy on the basis of Scripture. Luther objected to the charge, saying that Hus should have kept to the unity of the Church, but it became apparent that he had only a vague knowledge concerning what Hus had actually taught. At the lunch break, Luther retreated to the library to study documents from the Council of Constance: the assembly at which Hus had been condemned. As Luther studied Hus's words from the Council of Constance, he was surprised at what he found. Luther was especially surprised to find that Hus, even in taking actions that seemed to (further) fragment the institutional Catholic Church, actually quoted Augustine in affirming that there is only one Church, spiritually speaking, i.e., "the company of the predestined." When the debate continued, Luther affirmed, "Among the articles of John Hus, I find many which are plainly Christian and evangelical, which the universal Church cannot condemn." This statement greatly troubled Duke George, who viewed the Hussites as political enemies of the Saxons.
The Leipzig Disputation helped Luther to refine his doctrine of sola Scriptura: the understanding of Scripture alone as the final and sufficient authority concerning matters of life and godliness for the individual believer and the church as a whole. During the debate, Luther asserted:
[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. As for the pope's decretal on indulgences, 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. (Bainton, 90).