The Protestant Reformation was initiated in 1517 by Martin Luther, a German monk and theologian, who famously nailed his Ninety-Five Theses to the door of the Castle Church in Wittenberg. His theses challenged the Catholic Church's practices, particularly the sale of indulgences, and called for a return to what he believed were the pure teachings of Christianity based on the Bible. Luther’s actions sparked a movement that led to the establishment of various Protestant denominations and significant theological debates. The Reformation shifted the religious landscape of Europe, culminating in the fragmentation of Christianity and the rise of modern nation-states.