The central message of Martin Luther's 95 Theses, nailed to the door of the Wittenberg Castle Church in 1517, was a critique of the Catholic Church's practices, particularly the sale of indulgences. Luther argued that salvation is attained through faith alone, not through purchasing indulgences or church mediation. His theses challenged the authority of the Pope and the Church's interpretation of the Bible, advocating for a return to scripture and the belief that all Christians have direct access to God. This act is often seen as the catalyst for the Protestant Reformation.