In the 16th century, Europe was torn apart by an unprecedented wave of religious conflict that broke the more or less unified structure of Western Christianity into a variety of communions. This outline is designed to provide an overview of the religious turmoil of the Reformation.
The Reformation was made possible by the intricate politics of the early 1500s. The dominant figure was Emperor Charles V—“emperor” of Germany and nominal ruler of the independent-minded German nobility, but also the king of Spain, Austria, and Naples (which included all southern Italy) and the Duke of Milan. His only rival was François I of France, who took every opportunity to check his power. Other rulers, such as the popes and Henry VIII, tried to advance themselves by playing these two off of each other. Meanwhile, everyone was afraid of the Turks, who had conquered Eastern Europe and appeared capable of conquering all of Christendom.
Of course, the Reformation was primarily a religious movement, and its roots lie in the unprecedented growth of the Church between 1000 and 1300. During this period, its leaders tried to centralize as much power in Rome as possible, and to make religion significant for lay people and not only for the clerics and those who entered monasteries. However, their successes caused two problems: first, the popes needed a government to govern the church, so that the papal court rapidly came to look like an imperial capital than a spiritual center. Second, lay people wanted to live good lives but were still taught that God expected a level of virtue that ordinary people did not achieve. The idea of Purgatory, a place where imperfect people were purified after death, developed as a sort of compensation, and so did a collection of prayers, rituals, and objects that were supposed to save people who fell short. More devout people who tried to identify, regret, and pay for their sins were left on very shaky ground because there was no clear way to determine how much repentance was enough.
See:
Reformation History 1: 1517-1523
Reformation History 2: 1524-1531
Reformation History 3: 1531-1547
Reformation History 4: 1547-1570
Reformation History 5: 1570-1600
Reformation History Epilogue: 1600-1700