In the Middle Ages, the ultimate authority was pretty clearly God, and the pope was his mouthpiece. Heresy was not uncommon, of course, but excommunication and monastic imprisonment were the major punishments. Then, in 1215, came the Inquisition, and heresy was punishable by death. Spain in particular was a land of religious fanatics. Torquemada, the Grand Inquisitor of Spain from 1483 to 1498, made the Spanish Inquisition a household word.

Martin Luther (1483-1546) posted his 95 Theses (points of disagreement with the way the church was doing things) on the door of the castle church of Wittemburg. His Theses focused on the sale of indulgences and denied the primacy of the pope. He emphasized the idea that we are born in sin, our lack of free will, and our absolute need for the grace of God. Luther furthermore translated the Bible into German, and his dialect became the basis for standard literary German to the present day! He also wrote some pretty nasty papers condemning peasants and Jews.

John Calvin (Jean Cauvin, 1509-1564) from northern France, became Protestant and was forced to flee to Switzerland. There, he preached unconditional obedience to God and the odd doctrine of predestination, which says that since God is omniscient, he already knows who is going to heaven and who is not. Gaining political as well as spiritual power, he ruled Geneva as a religious dictatorship, not unlike Iran or Afghanistan recently: no drinking, dancing, or gambling; no icons, candles, or incense; obligatory church attendance for everyone.... He condemned the Spanish unitarian Michael Servetus, who came to him for protection, to burn at the stake for heresy! (A unitarian is someone who does not believe in the Trinity -- the worst heresy of all. Even today, the protestant churches of the US won’t accept the Unitarians as Christians! This despite the fact that the trinity is mentioned nowhere in the Bible.)

Henry VIII ruled England from 1509 to 1547. Having a hard time conceiving an heir, he divorced (and executed) one wife after another. When the pope refused to give him an easy divorce from Catherine of Aragon, he declared himself the head of the English Church and took all monastic property for his treasury! But the doctrines remained fundamentally Catholic. (Although usually considered Protestant, the Anglican Church and its offspring, the Episcopalian Church, maintain good relationships with the Catholic Church to this day.)

On the other hand, Philip II of Spain (ruled 1556-1598) wished to restore Catholicism to its former glory. His domestic policy consisted of encouraging the inquisition - resulting in the mass burning of heretics and severe oppression of remaining Moors and Jews in Spain.

Philip was especially ticked off by the war for independence, led by Protestants, of the Netherlands from Spain. Elizabeth I of England (ruled 1558-1603), whom he courted, secretly encouraged piracy against his fleets, which were coincidentally bringing loads of silver from the new world. The hostilities culminated in the destruction of his Great Armada in 1588.

The reformation led the Catholic church to reform itself, but not before executing a very large number of Protestants for heresy. The Protestants executed Catholics and other Protestants as well. Catholic or Protestant, these were not proud days for religion!