As the Passio tells it, Sebastian was a devout Christian who joined the Roman army — and rose to the Praetorian Guard under Diocletian and Maximian — precisely so that he could quietly strengthen persecuted believers without drawing suspicion (legend). He is said to have encouraged the imprisoned brothers Marcus and Marcellianus when they wavered, and to have converted many, including the prefect Chromatius and a woman named Zoe (legend). When his faith was exposed, Diocletian ordered him bound and shot by Mauretanian archers until, in the account's vivid phrase, he was as full of arrows as a hedgehog is full of spines (legend). Left for dead, he was found still breathing and nursed back to health by Irene, the widow of the martyr Castulus (legend). Recovered, he deliberately confronted the emperor a second time to reproach him — and this time was beaten to death with clubs, his body thrown into the Cloaca Maxima and later recovered for burial in the catacombs (legend).
Two further honest notes. First, the arrows: the earliest surviving image of Sebastian, a mosaic of about 682 in San Pietro in Vincoli, shows a bearded man in court dress with no arrows at all. The arrow-pierced youth is an invention of medieval and Renaissance art. Second, the plague: Sebastian became Europe's great plague-saint not through anything documented in his life but through a symbolic link — plague was imagined as arrows raining from heaven, and buboes resembled wounds — so the arrow-shot martyr was invoked against it, first at Rome in 680 and for centuries after. His fame, in other words, grew largely from a picture and a metaphor. The man beneath them remains almost entirely hidden, and the brand's task is not to pretend otherwise, but to honour the one thing that is sure: he died rather than deny Christ.