Martin Luther posted his 95 Theses in 1517, an event that ignited the Protestant Reformation. His theses criticized the Catholic Church's practices, particularly the sale of indulgences, which he viewed as corrupt and contrary to biblical teachings. Luther's act of nailing his theses to the church door in Wittenberg symbolizes the beginning of a significant religious and social upheaval in Europe. His call for reform inspired widespread debate and ultimately led to the establishment of various Protestant denominations, significantly altering the religious landscape of Europe and diminishing the power of the Catholic Church in both spiritual and political realms.