In 1520, Martin Luther's view of the papacy shifted dramatically and permanently. While the events of 1519 played a role in his evolving view of the papacy as the Antichrist forecast by St. Paul in 2 Thessalonians, those events alone cannot account for the suddenness and the totality of Luther's change of opinion. This essay argues that Lorenzo Valla's Discourse on the Forgery of the Alleged Donation of Constantine played a significant and too-little-appreciated role in Luther's new stance toward the papacy. This essay examines what it was about Valla's Discourse that helped convince Luther that the pope was the Antichrist.