Martin Luther is known for initiating the Protestant Reformation with his 95 Theses, which he famously posted on the door of the Castle Church in Wittenberg, Germany, in 1517. The Theses criticized the Catholic Church’s practice of selling indulgences and called for a return to biblical teachings and reforms within the Church. Luther’s actions sparked widespread religious debate and discontent within Europe, leading to the formation of various Protestant denominations and a significant shift in Christianity. His work also prompted the Catholic Church to initiate its own Counter-Reformation, fundamentally altering the religious and cultural landscape of Europe.