Graffito scratched on a plaster wall face of an ancient Roman school
Palatine Antiquarium, Rome

The Greek letters may be transcribed as ALEXAMENOS SEBETE THEON, "Alexamenos worships God." It is assumed that the comment is sarcastic: in what appears to be an attitude of prayer, the smaller figure stands before a crucified man with the head of an ass. Contemporary Christian writers remark that pagans accuse Christians of worshiping an ass.

In its discussion of the graffito (under "Archæology of the Cross and Crucifix"), the 1908 Catholic Encyclopedia suggests that the graffiti artist may have seen actual Christian worship involving a crucifix, because the figure on the cross is wearing the perizoma, the short loincloth which is commonly used in Christian images of the crucifixion.  (In actual crucifixions, the victim is naked.)

Here is a modern drawing of the graffito: