Martin Luther is the influential figure credited with initiating the Protestant Reformation in the early 16th century. In 1517, he posted his Ninety-Five Theses on the door of the Wittenberg Castle church, which challenged the Catholic Church's practices, particularly the sale of indulgences. Luther's critiques emphasized salvation through faith alone and the authority of scripture, leading to significant theological debates and the emergence of various Protestant denominations. His actions sparked widespread religious reform across Europe, altering the course of Christianity and influencing social and political structures, establishing a legacy that continues to resonate today.