Alexamenos.

A Christian mocked for believing in a crucified god.

Century III.

In the Catholic Encyclopedia, Archæology of the Cross and Crucifix, Orazio Marucchi writes:

On a beam in the Pædagogioum on the Palatine there was discovered a graffito on the plaster, showing a man with an ass's head, and clad in a perizoma (or short loin-cloth) and fastened to a crux immissa (regular Latin cross). Near by there is another man in an attitude of prayer with the legend Alexamenos sebetai theon, i.e., "Alexamenos adores God." This graffito is now to be seen in the Kircherian Museum in Rome, and is but an impious caricature in mockery of the Christian Alexamenos, drawn by one of his pagan comrades of the Pædagogioum.

Note that the word sebetai in the inscription as Marucchi gives it is actually a correction of what appears to be sebete. Rodney J. Decker of Baptist Bible Seminary has provided two photographs and a sketch of the graffito, whose inscription runs as follows in Greek:

ΑΛΕ-
ΞΑΜΕΝΟΣ
ΣΕΒΕΤΕ
ΘΕΟΝ.

Alexamenos, worship [plural] God.
Alexamenos worships God.

That first translation makes no sense; why the imperative should be in the plural is opaque. That second translation takes σεβετε as a phonetic misspelling of the middle σεβεται, which makes eminent sense: Alexamenos is worshipping his ass-headed god.

Tertullian confirms in To the Nations 11.1-2 that Christians were accused of worshipping the head of an ass (translation slightly modified from that at the Christian Classics Ethereal Library):

Nec tantum in hoc nomine rei desertae communis religionis, sed superductae monstruosae superstitionis. nam, ut quidam, somniasti{s} caput asininum esse deum nostrum; hanc Cornelius Tacitus suspicionem fecit. is enim in quarta historiarum suarum, ubi de bello Iudaico digerit, ab origine gentis exorsus, et tam de ipsa origine quam de nomine religionis, ut voluit, argumentatus, Iudaeos refert in expeditione vastis in locis {a}quae inopia laborantes onagris, qui de pastu aquam petituri aestimabantur, indicibus fontis usos evasisse; ita ob eam gratiam consimilis bestiae superficiem a Iudaeis coli.

In this matter we are [said to be] guilty not merely of forsaking the religion of the community, but of introducing a monstrous superstition; for some among you have dreamed that our god is the head of an ass, an absurdity which Cornelius Tacitus first suggested. In the fourth book* of his histories, where he is treating of the Jewish war, he begins his description with the origin of that nation, and gives his own views respecting both the origin and the name of their religion. He relates that the Jews, in their migration in the desert, when suffering for want of water, escaped by following for guides some wild asses, which they supposed to be going in quest of water after pasture, and that on this account the image of one of these animals was worshipped by the Jews.

* This is a mistake for the fifth book.

Tertullian writes almost exactly the same thing in Apology 16.1-2, and Minucius Felix agrees with him in Octavius 9 (English translation slightly modified from that of Roberts and Donaldson):

Ac iam, ut foecundius nequiora proveniunt, serpentibus in dies perditis moribus, per universum orbem sacraria ista teterrima impiae coitionis adolescunt. eruenda prorsus haec, et exsecranda consensio. occultis se notis et insignibus noscunt, et amant mutuo pene antequam noverint; passim etiam inter eos velut quaedam libidinum religio miscetur; ac se promisce appellant fratres et sorores, ut etiam non insolens stuprum, intercessione sacri nominis, fiat incestum; ita eorum vana et demens superstitio sceleribus gloriatur. nec de ipsis, nisi subsisteret veritas, maxima et varia maxime nefaria et honore praefanda, sagax fama loqueretur. audio eos turpissimae pecudis caput asini consecratum, inepta nescio qua persuasione, venerari; digna et nata religio talibus moribus. alii eos ferunt ipsius antistitis ac sacerdotis colere genitalia, et quasi parentis sui adorare naturam. nescio an falsa, certe occultis ac nocturnis sacris apposita suspicio: et qui hominem, summo supplicio pro facinore punitum, et crucis ligna feralia, eorum caerimonias fabulantur, congruentia perditis sceleratisque tribuit altaria, ut id colant quod merentur. iam de initiandis tirunculis fabula tam detestanda quam nota est: infans farre contectus, ut decipiat incautos, apponitur ei qui sacris imbuatur. is infans a tirunculo, farris superficie, quasi ad innoxios ictus provocato, coecis occultisque vulneribus occiditur.

And now, as wickeder things advance more fruitfully, and abandoned manners creep on day by day, those abominable shrines of an impious assembly are maturing themselves throughout the whole world. Assuredly this confederacy ought to be rooted out and execrated. They know one another by secret marks and insignia, and they love one another almost before they know one another. Everywhere also there is mingled among them a certain religion of lust, and they call one another promiscuously brothers and sisters, that even a not unusual debauchery may by the intervention of that sacred name become incestuous: it is thus that their vain and senseless superstition glories in crimes. Nor, concerning these things, would intelligent report speak of things so great and various, and requiring to be prefaced by an apology, unless truth were at the bottom of it. I hear that they adore the head of an ass, that basest of creatures, consecrated by I know not what silly persuasion, a worthy and appropriate religion for such manners. Some say that they worship the genitals of their pontiff and priest, and adore the nature, as it were, of their common parent. I know not whether these things are false; certainly suspicion is applicable to secret and nocturnal rites; and he who explains their ceremonies by reference to a man punished by extreme suffering for his wickedness, and to the deadly wood of the cross, appropriates fitting altars for reprobate and wicked men, that they may worship what they deserve. Now the story about the initiation of young novices is as much to be detested as it is well known. An infant covered over with meal, that it may deceive the unwary, is placed before him who is to be stained with their rites: this infant is slain by the young pupil, who has been urged on as if to harmless blows on the surface of the meal, with dark and secret wounds.

Tacitus, Histories 5.3-4a (English translation slightly modified from that of Church and Brodribb):

Plurimi auctores consentiunt orta per Aegyptum tabe quae corpora foedaret, regem Bocchorim adito Hammonis oraculo remedium petentem purgare regnum et id genus hominum ut invisum deis alias in terras avehere iussum. sic conquisitum collectumque vulgus, postquam vastis locis relictum sit, ceteris per lacrimas torpentibus, Moysen unum exulum monuisse ne quam deorum hominumve opem expectarent utrisque deserti, sed sibimet duce caelesti crederent, primo cuius auxilio praesentis miserias pepulissent. adsensere atque omnium ignari fortuitum iter incipiunt. sed nihil aeque quam inopia aquae fatigabat, iamque haud procul exitio totis campis procubuerant, cum grex asinorum agrestium e pastu in rupem nemore opacam concessit. secutus Moyses coniectura herbidi soli largas aquarum venas aperit. id levamen; et continuum sex dierum iter emensi septimo pulsis cultoribus obtinuere terras, in quis urbs et templum dicata.

Most writers, however, agree in stating that once a disease which horribly disfigured the body broke out over Egypt; that king Bocchoris, seeking a remedy, consulted the oracle of Hammon, and was bidden to cleanse his realm, and to convey into some foreign land this race detested by the gods. The people, who had been collected after diligent search, finding themselves left in a desert, sat for the most part in a stupor of grief, till one of the exiles, Moses by name, warned them not to look for any relief from God or man, forsaken as they were of both, but to trust to themselves, taking for their heavensent leader that man who should first help them to be quit of their present misery. They agreed, and in utter ignorance began to advance at random. Nothing, however, distressed them so much as the scarcity of water, and they had sunk ready to perish in all directions over the plain, when a herd of wild asses was seen to retire from their pasture to a rock shaded by trees. Moses followed them, and, guided by the appearance of a grassy spot, discovered an abundant spring of water. This furnished relief. After a continuous journey for six days, on the seventh they possessed themselves of a country, from which they expelled the inhabitants, and in which they founded a city and a temple.

Moyses quo sibi in posterum gentem firmaret, novos ritus contrariosque ceteris mortalibus indidit. profana illic omnia quae apud nos sacra, rursum concessa apud illos quae nobis incesta. effigiem animalis quo monstrante errorem sitimque depulerant.

Moses, wishing to secure for the future his authority over the nation, gave them a novel form of worship, opposed to all that is practised by other men. Things sacred with us have no sanctity with them, while they allow what with us is forbidden. In their holy place they have consecrated an image of the animal by whose guidance they found deliverance from their long and thirsty wanderings.