On October 31, 1517, a German monk named Martin Luther reportedly nailed his '95 Theses' to the door of the Castle Church in Wittenberg. Luther's primary grievance was the Catholic Church's practice of selling 'indulgences'—payments made to the church in exchange for the forgiveness of sins. His challenge to the Pope's authority sparked a theological revolution. Aided by the newly invented printing press, Luther's ideas spread rapidly across Europe, leading to the establishment of Protestantism and the fracturing of Western Christianity. This event triggered over a century of religious wars and fundamentally changed the political landscape of Europe, leading to an emphasis on individual conscience and the translation of the Bible into common languages.