Martin Luther wrote his 95 Theses in 1517 to protest the Catholic Church's practice of selling indulgences. He nailed the theses to the door of the Castle Church in Wittenberg, Germany, challenging the Church's authority. Luther argued that salvation comes through faith alone, not works like indulgences or other Church practices. The 95 Theses sparked the Protestant Reformation by disputing Catholic doctrine and enabling new Christian denominations to form.