The significant event that defined the start of the Reformation in Europe was Martin Luther's posting of the Ninety-Five Theses on the door of the Wittenberg Castle Church in 1517. Luther's document critiqued the Catholic Church's practices, particularly the sale of indulgences, sparking widespread debate and calls for reform. This act is often viewed as the catalyst for the Protestant Reformation, a religious movement that challenged the authority of the Catholic Church and led to the establishment of various Protestant denominations. Luther's actions emphasized the importance of individual interpretation of Scripture, transforming religious and political landscapes across Europe and altering the course of Christianity.