On January 3, 1521, Pope Leo X issues the papal bull Decet Romanum Pontificem, which excommunicates Martin Luther from the Catholic Church.

Martin Luther, the chief catalyst of Protestantism, was a professor of biblical interpretation at the University of Wittenberg in Germany when he drew up his 95 theses condemning the Catholic Church for its corrupt practice of selling indulgences, or the forgiveness of sins. He followed up the revolutionary work with equally controversial and groundbreaking theological works, and his fiery words set off religious reformers all across Europe.

In January 1521, Pope Leo X excommunicated Luther. Three months later, Luther was called to defend his beliefs before Holy Roman Emperor Charles V at the Diet of Worms, where he was famously defiant. For his refusal to recant his writings, the emperor declared him an outlaw and a heretic. Luther was protected by powerful German princes, however, and by his death in 1546, the course of Western civilization had been significantly altered.

READ MORE: Reformation: Definition and History