Martin Luther's Ninety-Five Theses, posted in 1517, are historically significant because they catalyzed the Protestant Reformation. Luther criticized the Catholic Church's practice of selling indulgences, arguing that salvation could not be bought and was achievable through faith alone. This document sparked widespread debate over church practices and doctrines, challenging the authority of the pope and promoting individual interpretation of the scriptures. The theses inspired reform movements that led to the establishment of various Protestant denominations, significantly altering the religious landscape of Europe and laying the groundwork for modern Christianity.