On October 31, 1517, an obscure German theology professor named Martin Luther sent a list of ninety-five academic complaints about church corruption to his archbishop, Albrecht of Mainz — who happened to be selling the very indulgences Luther attacked, to repay a debt to the Fugger bank. He may also have nailed them to the Wittenberg castle church door; that detail is a later embellishment. Within three years his arguments were printed across Europe, he was excommunicated by Leo X and outlawed by the emperor at the 1521 Diet of Worms, and a continent that had been one religious civilization for a thousand years was beginning to split apart along a fault that would not heal.
The substance of Luther's complaint was technical — about indulgences, the sale of forgiveness, the authority of the pope to free souls from purgatory. The consequence was structural. Luther's argument that scripture, not the church, was the final authority (sola scriptura), and that salvation came through faith alone, dynamited the medieval clergy's monopoly on access to God: if any literate believer could read the Bible directly — and Luther's 1522 German translation let them — the priesthood lost its indispensability. Within a generation there were Lutherans, Calvinists in Geneva, Anglicans after Henry VIII's 1534 break with Rome, Anabaptists hunted by everyone, each with its own theology, each backed by a prince who gained control of church lands and clerical taxation by converting — which is why the Reformation spread fastest where rulers had the most to gain from breaking with Rome. The ensuing wars of religion, from the German conflicts settled at Augsburg in 1555 to the catastrophic Thirty Years' War, killed millions and depopulated parts of central Europe, lasting with intermissions until the 1648 Peace of Westphalia. The longer-term effects were more interesting: literacy rates rose where Protestantism took hold (you needed to read the Bible yourself); the modern individual conscience — the idea that you are responsible for your own salvation — became a default of European thought; and the eventual exhaustion produced the secular state, which kept the peace precisely by declining to enforce any creed.
Almost every modern argument about religious authority, individual conscience, and the proper place of faith in public life is downstream of October 1517. The Reformation also installed, perhaps permanently, the European reflex of splitting when an institution loses legitimacy — schism rather than reform from within. We watch the same dynamic whenever a movement, a party, or a church fractures into rival orthodoxies, each certain it holds the original truth: the template was set five centuries ago by a monk who only meant to start a debate.