Father Delehaye classes the story of St. Genesius in the category of imaginative romances. It is possible that Genesius never existed at all, but is a western version of St. Gelasius of Heliopolis, of whom (and of others) a similar tale is told. The legend of Genesius is narrated by Alban Butler as follows.
The Emperor Diocletian coming to Rome, he was received with great rejoicings. Among other entertainments prepared for him, those of the stage were not neglected. In a comedy which was acted in his presence one of the players took it into his head to burlesque the ceremonies of Christian baptism, which could not fail to amuse the people, who held our religion and its mysteries in contempt and derision. This player therefore, whose name was Genesius and who had learned some things concerning Christian rites from friends who professed that religion, laid himself down on the stage, pretending to be ill, and said, “Ah my friends, there is a great weight upon me, and I would gladly be eased.” The others answered, “What shall we do to give you ease? Would you like us to plane you and reduce the weight that way?” “Idiots!” he exclaimed, “I am resolved to die a Christian, that God may receive me on this day of my death as one who seeks His salvation by turning from idolatry and superstition.”
Then a priest and exorcist were called, that is to say, two players who impersonated these characters. These, sitting down by his bedside, asked, “Well, my child, why did you send for us?” But here Genesius was suddenly converted by a divine inspiration and replied, not in mockery but seriously, “Because I desire to receive the grace of Jesus Christ and to be born again, that I may be delivered from my sins.”
The other players then went through the whole ceremony of baptism with him; but he in earnest answered the usual interrogatories, and on being baptized was clothed with a white garment. After this, other players, dressed like soldiers, to carry on the jest, seized him and presented him to the emperor, to be examined as the martyrs were wont to be. Genesius then declared himself openly and seriously, standing upon the stage, “Hear! O emperor, and all you that are present, officers, philosophers, senators and people, hear what I am going to say. I never yet so much as heard the word Christian but I reviled it, and I detested my very relations because they professed that religion.
“I learned its rites and mysteries only that I might the better ridicule it, and inspire you with the utmost contempt for it; but when I was to be washed with the water and examined, I had no sooner answered sincerely that I believed, than I saw a company of angels over my head, who recited out of a book all the sins I had committed from my childhood and having plunged the book into the water which had been poured upon me in your presence, they showed me the book whiter than snow. Wherefore I advise you, O great and mighty emperor, and all people here present who have mocked these mysteries, to believe with me that Jesus Christ is the true Lord; that He is the light and the truth; and that it is through Him you may obtain the forgiveness of your sins.”
(Assuming the story to be true, the “baptism” administered would not be valid, for lack, on the part of the sacrilegious actor, of any intention even “to do what the Church does” when she baptizes. Genesius received the baptism, not of water, but of desire and of blood.)
Diocletian, enraged at these words, ordered him to be beaten, and afterward to be put into the hands of Plautian, the prefect of the praetorium, that he might compel him to sacrifice. Plautian put him upon the rack, where he was torn with iron hooks and then burnt with torches; but the martyr persisted in crying out, “There is no other Lord beside Him whom I have seen. Him I worship and serve, and to Him I will cling, though I should suffer a thousand deaths. No torments shall remove Jesus Christ from my heart and my mouth. Bitterly do I regret that I once detested His holy name, and came so late to His service.” At length his head was struck off.