Outside legends and hypothesized origins (such as the Mandylion of Edessa and the Shroud of Turin), the earliest proven image is a piece of anti-Christian graffiti on the side of a school in Rome, of uncertain but early date.

It shows a human figure hanging on a cross with a donkey's head; by the side is someone raising his arm to the figure. The caption says, "Αλεξαμενος ϲεβετε θεον," or, "Alexamenos worships his god." Since the god in question is hanging on a cross, most scholars accept this to be a depiction of Christ being worshiped.