Calas, along with his wife, was a Protestant. France was then a mostly Catholic country; Catholicism was the state religion. While the harsh repression of Protestantism initiated by King Louis XIV had largely receded, Protestants were, at best, tolerated. Louis, one of the Calas' sons, converted to Catholicism in 1756. On October 13-October 14, 1761, another of the Calas' sons, Marc-Antoine, was found dead on the ground floor of the family's home. Rumors had it that Jean Calas had killed his son because he, too, intended to convert to Catholicism. The family, interrogated, first claimed that Marc-Antoine had been killed by a murderer. Then they declared that they had found Marc-Antoine dead, hanged; since suicide was then considered a heinous crime against oneself, and the dead bodies of suicides were defiled, they had arranged for their son's suicide to look like a murder.
On March 9, 1762, the parlement (appellate court) of Toulouse sentenced Jean Calas to death on the wheel. On March 10, he died tortured on the wheel, while still very firmly claiming his innocence. Voltaire, contacted about the case, after initial suspicions that Calas was guilty of anti-Catholic fanaticism had subsided, began a campaign to get Calas' sentence overturned.