It was not until April 1518, a half year after the publication of the theses, that Luther was invited to Heidelberg for a debate. There he did not touch the matter of indulgences.

Luther's point was that religion...teaches exactly the opposite of the gospel of Jesus Christ.

Rather, he confronted the teachings that had made indulgences possible in the first place. Luther presided over the discussion. According to the custom of that day, one of his students presented Luther's position. He stated that the law of God, though it is certainly good and true, cannot make people good. Even if one could fulfill the law, it would have no effect on the goodness of his nature. All human works are sins, as long as the person performing them is a sinner.

Luther himself did not think that this was such a novel discovery. Jesus had frequently said that a good tree bears good fruit and a bad tree evil fruit. People cannot make themselves into good trees by decorating themselves with good fruit. Only God can make a tree good.... [17]

Luther's point was that religion, in making God into a referee who hands out rewards and penalties based on men's works, teaches exactly the opposite of the gospel of Jesus Christ. Good works cannot make a man good, and only those with a new nature can do genuinely good works.

John Eck was paying close attention to all that Luther was doing. Eck, formerly Luther's personal friend, was a professor of theology at the University of Ingolstadt. As a forceful debater with a clear-sighted reasoning ability, Eck became the most capable opposer of Luther that Germany produced. As he studied the ninety-five theses, he saw that the principles behind them would justify the teachings of John Hus, which had come to be called “the Bohemian Heresy.” Furthermore, if carried to their logical conclusion, the theses would undermine the supposed supernatural power of the clergy as well as their dominion over the laity, thus destroying the whole structure of the Roman Catholic Church.

Hence, a second debate was arranged by Carlstadt, an older colleague of Luther's,

Midway into the debate, Luther jumped into the battle...

to take place in Leipzig in the summer of 1519. Carlstadt and Eck were to debate, with Luther and Melanchthon attending as spectators. The three professors from Wittenberg, Carlstadt, Luther, and Melanchthon, traveled in two ordinary carts accompanied by two hundred students on foot. The students went along to show their support and carried spears in order to defend their teachers, if necessary.

Midway into the debate, Luther jumped into the battle, not satisfied with the progress. Now Luther and Eck stood face to face. One onlooker gave the following description of Luther:

Martin is of medium height; his body is slender, emaciated by cares and study; one can count almost all the bones; he stands in the prime of his age; his voice sounds clear and distinct. [18]

He also described Eck:

He has a huge square body, a full strong voice coming from his chest, fit for a tragic actor or a town crier, and more harsh than distinct; his mouth, eyes, and whole aspect give one the idea of a butcher or a rude soldier rather than of a theologian. [19]

The debate raged back and forth. Eck insisted that the pope is the head of the church on earth. Luther said that Jesus Christ is the only head the church has and needs. He shocked the listeners by declaring that not only the pope but even the church councils can err. He cited the council of Constance as having condemned some very Christian and evangelical teachings in its condemnation of John Hus. Under the pressure of the debate, Luther said, “Dear doctor, the Hussite opinions are not all wrong.” Upon hearing this, a duke in the audience cried out, “God help us! The pestilence!”

Having forced Luther to associate himself publicly with an excommunicated arch-heretic, Eck felt that the battle had been won. At the same time, however, Luther became known as an unyielding opponent of Rome, a champion of all who longed to see a “Germany for the Germans.” [20]

He discovered that his teachings involved much more than a simple protest against some abuses of the church.

The Leipzig Disputation was crucial to Luther's development, for it helped him to see clearly why he was so opposed to the indulgences. He discovered that his teachings involved much more than a simple protest against some abuses of the church. This revelation destroyed within Luther any basis for the worship of saints, the reverence of relics, and useless religious pilgrimages.

He saw more than ever before the sharp contrast between the free pardoning grace of God and the monastic life with all of its vigils, fasts, scourgings, and mortifications of earthly and family affections. Three important matters in particular became clear to Luther in this debate: 1) The distinction between the law and the gospel. The law can only condemn people, but through the preaching of the gospel they can be saved. 2) All men and all institutions can and do err. 3) Jesus Christ is the sole Head of the church. Men must obey Christ and His Word. [21]