Luther, among other things, thought it was wrong for the Catholic Church to sell pieces of paper called "Indulgences," which were ostensibly designed to shorten a soul’s stay in purgatory. (Indulgences can still be obtained from the Catholic Church, but they must be earned, not purchased.)

Since the Church used money obtained from the sale of "Indulgences" to fund projects (like St. Peter’s Basilica), Luther (who later married Katharina von Bora with whom he had six children) believed the Church was deceiving people. He argued that mankind was saved by sola Scriptura (Scripture - the Bible - alone) and sola fide (faith alone), not by the teachings of the Catholic Church.

With his 95 Theses, citing Church abuses, Luther (follow the link to see his home in Wittenberg) started a protest against the authority of the Catholic Church. His reforms led to the Protestant Reformation; his ideas quickly spread throughout Europe.

John Calvin, in Switzerland, agreed with Luther. His ideas, set forth in Institutes of the Christian Religion, spread to France. By 1550, preachers brought Bibles from Switzerland into France where people converted to the Protestant faith in astonishing numbers. They were called Huguenots.

French Catholics believed that Protestant ideas would cause the wrath of God to descend on everyone. To avoid such a catastrophe, would they have to wipe out the Protestant faith - and its adherents - from French soil?

Protestants, on the other hand, thought Catholics were misguided. With unrestrained arrogance, Huguenots destroyed relics Catholics held sacred even as they ruined crosses and statues of saints.

One powerful French noble family (Bourbon) was Protestant while another (Guise) was Catholic. (Mary Queen of Scots, a member of the Guise family, was married to the young French King Francis II, son of Catherine de Medici.)

Each side believed its view was "truth." And Luther, near the end of his life, became deeply antisemitic. The stage was set for war and massacres.