What is known as the Protestant Reformation was a 16th century religious movement that sought to reform the Roman Catholic Church. Leaders of the movement included Martin Luther, John Calvin, John Knox, Ulrich Zwingli and others. The complaints against the Catholics led to the establishment of Protestantism within the world of Christianity.
Father of Reformation
Luther (1483 to 1546 A.D.) is considered the "father" of the Reformation. The start of the Protestant revolution began on Halloween (All Hallow's Eve - October 31) in 1517 A.D. when he nailed his famous "95 Theses" to the door of a Catholic church. He also, on the same day, mailed a copy of his complaints to the archbishop.
Luther criticized the Catholic Church for their practice of selling indulgences (begun when Sixtus was Pope from 1471 - 1484) and for their views on purgatory and papal authority. He was ultimately excommunicated from the church in 1521.
Interestingly, according to the Catholics, Martin Luther nailed his complaints not on a plain church door but on one that was considered the bulletin board of the university where notices and disputations were frequently posted (1913 Catholic Encyclopedia article on Martin Luther).
The growing discontent with Catholicism during the 16th century also led to a bloody series of religious wars that culminated in the Thirty Years' War (1618 - 1648 A.D.). During this period of fighting somewhere between 25 and 40% of Germany's population was decimated. The Protestant Reformation itself, begun in 1517, is believed to have ended around 1750 A.D.
Protestant churches and movements that came directly out of the Reformation include the Lutherans, the Reformed (Calvinist) church, the Church of England, the Anabaptists (from which the Amish and Mennonite churches trace their history), the Church of Scotland, and others.