In 1761, in Toulouse, there lived a young man named Marc-Antoine Calas, the son of a merchant named Jean. The father and son, like the rest of the family, except for one Catholic brother, were Huguenots, at a time when intolerance and fanaticism were still strong in France.

Marc-Antoine, already unhappy due to his professional failures, and having lost all his money to gambling, decided to take his own life. One evening, after dinner, his family and a friend found him hanged on the ground floor of their home. The discovery of the body caused great agitation among the people of Toulouse. Soon, rumors began to spread that the young man had been murdered by his father and family because he had told them he intended to convert to Catholicism the following day.

This was baseless: even if Marc-Antoine had wanted to become Catholic, his father would not have objected. Jean Calas had already allowed one of his other sons to convert, and he even employed a Catholic servant. He was certainly not a religious fanatic.

Nevertheless, the townspeople became convinced that Marc-Antoine was a martyr of the Catholic faith, killed in odium fidei by heretics. The local authorities and religious confraternities began treating him as such immediately after his death. The capitoul (magistrate) of Toulouse, David de Baudrigue, after a solemn ceremony, had him buried in the church of Saint-Étienne, while a local religious confraternity organized a procession in his honor, displaying in a catafalque a skeleton (representing Marc-Antoine) holding a pen with which he was supposedly going to sign his abjuration of Protestantism the next day.

Very quickly, strong devotion grew around his figure: his teeth began to circulate as relics, a young man prayed at his tomb every night, and stories of miracles attributed to his intercession spread.

His father, Jean Calas, considered the chief culprit in his alleged murder, was summarily tried in 1762, tortured, and burned at the stake. His family obtained justice only in 1765, when King Louis XV declared Jean Calas innocent and dismissed the magistrate who had condemned him.

This tragic and absurd story of a young Huguenot who committed suicide, but was mistaken by the people of Toulouse for a Catholic martyr, inspired the philosopher Voltaire to write his famous Treatise on Tolerance.