At the start of the 16th century the Roman Church stood at the center of European religious, economic and political life. Indulgences sold to fund the rebuilding of St Peter's Basilica — documents claimed to shorten the time the souls of dead relatives would spend in purgatory — became a public scandal under the aggressive marketing of the Dominican preacher Johann Tetzel in Germany. Martin Luther, an Augustinian friar and professor of theology at the University of Wittenberg, sent ninety-five Latin theses challenging this practice to the Archbishop of Mainz on 31 October 1517. The famous image of Luther nailing the theses to the door of the Schlosskirche is a later addition and historians still debate whether it actually happened.

The movable-type press that Gutenberg had introduced around 1440 now became the catalyst of the Reformation. The 95 Theses began as an academic manuscript; once translated into German and printed, they spread across the Empire within weeks. Luther's later works — 'On the Freedom of a Christian', 'To the Christian Nobility of the German Nation' — sold hundreds of thousands of copies and produced the first true 'media campaign' in history. Refusing to recant before the Diet of Worms in 1521, Luther was outlawed by Emperor Charles V; under the protection of Frederick the Wise of Saxony he translated the Bible into German. In Zurich Huldrych Zwingli and in Geneva John Calvin pursued their own paths of reform; 'sola scriptura' (Scripture alone) and 'sola fide' (faith alone) became the shared roof over diverging branches.

The consequences were not only religious. The princes of the German-speaking lands turned confessional choice into a tool of political autonomy; the 1555 Peace of Augsburg accepted the principle 'cuius regio, eius religio' (whose region, his religion). Vernacular reading raised literacy, expanded the book market, and led to the creation of local church structures that replaced papal authority. But the confessional split also opened a destructive cycle of war — within a century Europe would be torn by the religious wars that culminated in the Thirty Years' War.

The long-term legacy of the Reformation was the end of a single institutional authority's right to define Christian faith. This planted in Europe the seeds of 'individual conscience', and later of secularization, constitutional limits, and the conceptual tools needed for different confessions to share the same territory. The Peace of Westphalia (1648) and the Enlightenment debates on toleration descend directly from this rupture. 1517 was not a piece of paper nailed to a door; it was the starting date of Europe's exit from a single-authority worldview.