Why did Western Christianity split in the 1500s?

Western Christianity split because a dispute over indulgences grew into a dispute over authority itself: who decides what Christians must believe, the Church or the Bible? In 1517, a Wittenberg professor, Martin Luther, wrote the Ninety-five Theses, propositions for an academic debate challenging how indulgences, certificates believed to reduce punishment in purgatory, were being sold.

The trigger was a fundraising campaign. In 1515 Pope Leo X had granted an indulgence to finance the new St. Peter's Basilica in Rome, and in 1517 the preacher Johann Tetzel sold it near Wittenberg. Luther was alarmed when his parishioners came home claiming they no longer needed to repent. His theses mocked the idea that as soon as a coin clinks in the chest, a soul flies out of purgatory, a line that reflected Tetzel's exaggeration rather than official teaching.

The famous image of Luther hammering the theses to a church door may be partly legend. He sent them to his archbishop on 31 October 1517; the story of the door comes later, from his colleague Melanchthon, and he may never have posted them there at all.

The printing press spread the argument fast. At the Diet of Worms in 1521, Luther refused to recant and was declared an outlaw. Other reformers followed, the Catholic Church answered with the Council of Trent, and in 1555 the Peace of Augsburg let each German prince choose his realm's faith. Luther later said he might not have started had he known where it would lead.