French Calvinists were known as Huguenots. There is no definitive information on the origin of the term. John Calvin himself had been a French humanist, although he had been forced to flee to Geneva and promulgated his theological ideas from there. His ideas met with wide acceptance in France, and Calvinism became quite popular. As had been the case in Germany, it appealed to a number of noblemen who saw in it an opportunity to free themselves from the power of the King, who had the authority to appoint Church officials under the Concordat of Bologne. The Concordat had made Catholicism the French state religion. However, the French monarchy remained staunchly Catholic, and the Huguenots were repressed and persecuted as heretics.
Calvinism enjoyed a brief respite under the reign of Francis II, whose mother, Catherine de Medici ruled as his regent. Catherine attempted to strike a balance between Catholic and Calvinist factions. An arrangement was made for Francis' sister to marry Henry of Navarre, a Huguenot prince, which everyone believed would resolve the dispute. A great celebration was scheduled to mark the marriage, and Huguenots were encouraged to attend, but not to bring their weapons. This turned out to be a trap; thousands of Calvinists were slaughtered in the St Bartholomew Day's Massacre.
Henry of Navarre, the Calvinist, ultimately became King Henry IV after the War of the Three Henries. Henry's religious convictions did not run deep and after famously stating that "Paris is worth a mass," converted to Catholicism. He did so to prevent further violence. In 1598, he issued the Edict of Nantes which granted religious freedom to the Huguenots and allowed them the right to use schools and other public institutions just as Catholics. The Edict remained in effect until it was revoked by Louis XIV.
Calvinism played a major role in politics in the France in the 16th century. The conflict between Calvinism and Catholicism led to civil wars that were based on religion.
The French monarchy was Catholic and tried hard to stop the spread of Calvinism. However, it was unsuccessful and large numbers of French people embraced Calvinism. This included many merchants and other wealthy people as well as close to half of the nobility. Overall, the Calvinists constituted only about 10% of the population, but they were well-organized and very politically active. The presence of the Calvinists, combined with the unhappiness that many French people felt about the crown's attempts to centralize power, led to conflict and eventually to a series of civil wars.