Martin Luther was a German monk and theologian who lived in the 16th century. He is best known for challenging the practices of the Catholic Church, particularly the sale of indulgences, which were payments made to reduce punishment for sins. In 1517, he famously nailed his Ninety-Five Theses to the church door in Wittenberg, sparking the Protestant Reformation. Luther's ideas emphasized faith in God and the importance of the Bible as the sole source of religious authority. His teachings led to the formation of various Protestant denominations and changed the course of Christianity, promoting the idea that individuals could have a direct relationship with God without the need for priests.