Martin Luther posted the 95 Theses on the door of the Castle Church in Wittenberg,  

Germany on October 31, 1517. 

The 95 Theses were a series of propositions for debate that questioned the Catholic Church's practices and beliefs, particularly the sale of indulgences. Luther intended to debate the theses with church leaders and anyone else who was interested. 

The 95 Theses became the foundation of the Protestant Reformation. The theses were quickly translated into German and other languages and spread throughout Europe. The conflict between Luther and the Catholic Church became a media event, and Luther's supporters became some of the first "bestsellers" in history. 

Luther was excommunicated from the Catholic Church in 1521. The term "Protestant" first appeared in 1529 when supporters of Luther protested the Edict of Worms. 

The 95 Theses are considered the beginning of the Protestant Reformation, and the anniversary of their posting is celebrated annually as Reformation Day.