The central character is not actually either of the Saints named Denis, whose feast is October 10, but Abbot Hilduin of Saint-Denis (in Paris). He took the story of Dionysus the Areopagite, an Athenian leader who was converted by St. Paul's preaching, and Denis, a third century Christian who was sent from Italy to establish a missionary center in Gaul. Dionysus became bishop of Athens and was eventually martyred; Denis was recognized as bishop of Paris and also martyred. Eager for to boost the pilgrim trade, Hilduin bent their stories a little, so that Dionysus left Athens for Paris, and the bones in the reliquary of Saint-Denis was from a man actually converted by the Apostle Paul himself.
Religious fraud of this kind is a tricky business. I got softer on it after reading A Prayer for Owen Meany. True, the central character did not profit personally from the fraud, but if I can paste my own layer over this story (truthiness alert), Hilduin was motivated by a desire to raise funds to feed the poor and glorify the Lord, not to fill his belly and aggrandize his abbey. With all the invention of saints floating around, I can't blame a dog for barking.