The event that signified the beginning of the Protestant Reformation was Martin Luther's posting of the Ninety-Five Theses on October 31, 1517. This document challenged the practices of the Catholic Church, particularly the sale of indulgences, and questioned the authority of the Pope. Luther's actions sparked a widespread movement aimed at reforming the church’s doctrines and practices. The Reformation led to the establishment of various Protestant denominations and significant religious, political, and social changes across Europe, eventually reshaping Christianity and leading to conflicts such as the Thirty Years' War.