The event that marked the start of the Protestant Reformation was Martin Luther's posting of the Ninety-Five Theses in 1517. Luther, a German monk and theologian, challenged the Catholic Church's practices, particularly the sale of indulgences, which absolved individuals of sin in exchange for money. His theses criticized various church doctrines and practices, sparking debate and discussion across Europe. The publication of the Ninety-Five Theses is considered a pivotal moment in history, leading to the establishment of Protestant denominations and significant changes within Christianity. Luther's actions initiated a movement that would have lasting impacts on religion and society.