Martin Luther (1483–1546) was a German priest, theologian, and a seminal figure in Christian history who initiated the Protestant Reformation. He is best known for challenging several teachings and practices of the Roman Catholic Church, particularly the sale of indulgences, which he criticized in his Ninety-five Theses in 1517. His theological ideas, emphasizing salvation by faith alone and the authority of Scripture over church hierarchy, led to his excommunication and condemnation as an outlaw by the Holy Roman Emperor. Luther's beliefs laid the foundation for Lutheranism, a major branch of Protestant Christianity, and his actions sparked religious, political, and social change in Europe.