Sources: Luther, 95 Theses (1517); Luther, On the Babylonian Captivity of the Church (1520); Calvin, Institutes of the Christian Religion (1536–59); Zwingli, Commentary on True and False Religion (1525); the Augsburg Confession (1530).

The Protestant Reformation begins October 31, 1517 when Martin Luther nails (or posts) his 95 Theses challenging the sale of indulgences to the door of the Castle Church in Wittenberg. The deeper causes: Renaissance humanism’s return to primary sources (ad fontes) enabling textual criticism of the Vulgate; Erasmus’s Greek New Testament (1516) revealing translation errors; widespread corruption in the institutional church; printing press democratizing access to Scripture. Luther’s core theological insights: (1) Sola fide — justification by faith alone, not works; (2) Sola scriptura — Scripture alone as the supreme authority, over tradition and councils; (3) The priesthood of all believers — direct access to God for all Christians, no sacerdotal mediator required; (4) Simul justus et peccator — the believer is simultaneously righteous (in Christ) and sinful. Calvin adds: double predestination (God elects some to salvation and reprobates others to damnation), covenant theology, and a more systematic ecclesiology. The Reformation fractures Western Christianity permanently: Lutheran, Reformed/Calvinist, Anglican, Anabaptist — each with different ecclesiologies and theologies of sacraments, authority, and salvation.