From the Treasury of Daily Prayer:

On October 31, 1517, an Augustinian monk posted ninety-five statements for discussion on the door of the Castle Church in Wittenberg, Germany. Dr. Martin Luther hoped that posting his theses would bring about an academic debate regarding repentance, the sale of indulgences, and other matters of concern within the Roman Catholic Church. However, Rome eventually excommunicated Luther, judging him to be a heretic. Luther’s reforms, centered on the teaching that a believer is justified by grace through faith in Jesus Christ, sparked religious reforms not only in the German states but also in many European countries. In 1667, Elector John George II of Saxony standardized the custom of observing Luther’s October 31 posting of the Ninety-five Theses.