Why was the Leipzig Disputation with Johann von Eck an important turning point in Martin Luther's religious development?
The Leipzig Disputation took place at Pleissenburg Castle in Leipzig, began in June 1519, ended in July, and lasted three weeks, only two years after Martin Luther posted his famous Ninety-Five Theses to the door of All Saint's Church in Wittenberg in 1917. The famous theses protested against clerical abuses and the sale of indulgences and launched the Reformation. Initially, Dr. Johann Maier von Eck, an academic theologian and supporter of Catholicism, had invited Andreas Karlstadt to debate theological doctrines on free will and grace, but then Eck invited Luther as well, and the debate expanded to include questions of purgatory, the sale of indulgences, penance, and the authority of the pope. The debate essentially ended with both sides feeling they had won, while Luther felt the whole debate had been a waste of time. However, the reason why the theological disputation was historically important is because it was the first time Luther "denied the divine right and the origin of the papacy," as well as "the infallibility of the general council" ("The Leipzig Disputation"). Once Luther made a denouncement of the authority and infallibility of the pope, the Church's only ruler, naturally a schism was only inevitable, leading to the Reformation and the creation of the Protestant church.
With respect to the infallibility of the pope, Dr. Eck argued what is still a Catholic doctrinal belief today, that the pope is "the successor of Peter, and the vicar [or religious representative] of Christ by divine right." Luther argued, in contrast, that the belief in papal primacy is "contrary to the Scriptures, the ancient church, to the Council of Nicaea,--the most sacred of all Councils,--and rests only on the frigid decrees of the Roman pontiffs" ("The Leipzig Disputation"). Essentially, Luther saw that if the pope and the priests could behave corruptly, such as by selling indulgences, then even the pope and priests could be wrong, proving that the pope is not actually infallible.
Hence, while Luther failed to get Dr. Eck and his supporters to agree with him, his argument against the divine authority and infallibility of the pope stated during the Leipzig Disputation certainly did serve as a turning point by making a schism in the Catholic Church and the Reformation inevitable.