The trouble in France started after Martin Luther, at the time a German monk, nailed 95 arguments with Rome on the door of the Castle Church in Wittenberg, Germany. It was October 31, 1517.
Luther, among other things, thought it was wrong for the Catholic Church to sell "Indulgences" (a piece of paper ostensibly designed to shorten a soul’s stay in purgatory).
Since the Church used money obtained from the sale of "Indulgences" to fund projects (like St. Peter’s Basilica), Luther 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." The stage was set for war and massacres.