The legendary account of the double martyrdom of Saint Marcel and Saint Anastasius is the first instance of evangelical activity in Argentomagus. The coming of these two apostles of Christian faith is traditionally put at around the middle of the 3rd century in the reign of the emperor Decius (248 - 251).

According to legend, Marcel was only 15 years old although Anastasius seems to have been middle-aged. Coming from Rome and heading towards Toulouse, the two missionaries stopped at a house in a suburb of Argentomagus. There Marcel produced a first miracle by giving health to a child who was blind, deaf, dumb and lame. Then, in a recreation of the miracle from the Wedding in Canaan, he proceeded to transform water into wine much to the amazement of the assembled neighbourhood.

Learning of the foment in the area after these two miracles, Heracles, the praetor of the town, soon summoned this miracle-worker and his companion to appear before him and renounce their faith.

Irritated by their refusal to worship Apollo, Diana and Hercules, the gods venerated in the temple, Heracles ordered Marcel to be tortured firstly on the rack and then on the grill over glowing embers. The young man withstood all these trials before demanding to be led to the entrance of the sanctuary. There, before a considerable crowd, Marcel ordered Apollo to come out of the temple. The god did as he was bade and, with a great roar, evaporated in a cloud of sulphur. Then Marcel entered the temple, at which point all the statues of the idols fell from their pedestals and broke at his feet.

Having been tortured once more, Marcel was thrown into a dungeon and was shortly afterwards decapitated although not before he had prophesied the manner of his death. Tradition has it that in fact Saint Marcel was martyred and buried on the site of the current church. As to his companion, he was put to death on the rack in the place known as Clos Saint-Anastasius, today the Champ de l'Image.

Whatever happened, archaeology is of no help here since no trace of Christian symbolism has ever been discovered amongst the Gallo-Roman artefacts.