The Protestant Reformation began on October 31, 1517, when Martin Luther, a German monk, 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 for the remission of sins. He argued that salvation was achieved through faith alone and that the Bible, rather than the Pope, was the ultimate religious authority. Aided by the printing press, Luther's ideas spread rapidly, leading to the fragmentation of Western Christianity into various Protestant denominations. The Reformation sparked a century of religious wars, including the Thirty Years' War, and prompted the Catholic Counter-Reformation. Beyond religion, it encouraged individual literacy, challenged the absolute authority of monarchs, and influenced the development of modern nationalism and the separation of church and state.