The Protestant Reformation in the 16th century was initiated by Martin Luther, a German theologian and monk. Luther's critical stance on several practices of the Catholic Church, particularly the sale of indulgences, led him to post his "95 Theses" in 1517. This act challenged the authority of the Church and called for a return to biblical teachings. The movement sparked significant religious, political, and cultural upheaval in Europe, resulting in the establishment of various Protestant denominations and altering the course of Christianity, as well as impacting social and political structures in the continent.