For, like some others, you are under the delusion that our god is an ass's head. Cornelius Tacitus first put this notion into people's minds. In the fifth book of his histories, beginning the (narrative of the) Jewish war with an account of the origin of the nation; and theorizing at his pleasure about the origin, as well as the name and the religion of the Jews, he states that having been delivered, or rather, in his opinion, expelled from Egypt, in crossing the vast plains of Arabia, where water is so scanty, they were in extremity from thirst; but taking the guidance of the wild asses, which it was thought might be seeking water after feeding, they discovered a fountain, and thereupon in their gratitude they consecrated a head of this species of animal.
And as Christianity is nearly allied to Judaism, from this, I suppose, it was taken for granted that we too are devoted to the worship of the same image. But the said Cornelius Tacitus (the very opposite of tacit in telling lies) informs us in the work already mentioned, that when Cneius Pompeius captured Jerusalem, he entered the temple to see the arcana of the Jewish religion, but found no image there. Yet surely if worship was rendered to any visible object, the very place for its exhibition would be the shrine; and that all the more that the worship, however unreasonable, had no need there to fear outside beholders.
For entrance to the holy place was permitted to the priests alone, while all vision was forbidden to others by an outspread curtain. You will not, however, deny that all beasts of burden, and not parts of them, but the animals entire, are with their goddess Epona objects of worship with you. It is this, perhaps, which displeases you in us, that while your worship here is universal, we do homage only to the ass. Then, if any of you think we render superstitious adoration to the cross, in that adoration he is sharer with us.
If you offer homage to a piece of wood at all, it matters little what it is like when the substance is the same: it is of no consequence the form, if you have the very body of the god. And yet how far does the Athenian Pallas differ from the stock of the cross, or the Pharian Ceres as she is put up uncarved to sale, a mere rough stake and piece of shapeless wood? Every stake fixed in an upright position is a portion of the cross; we render our adoration, if you will have it so, to a god entire and complete.
We have shown before that your deities are derived from shapes modelled from the cross. But you also worship victories, for in your trophies the cross is the heart of the trophy. The camp religion of the Romans is all through a worship of the standards, a setting the standards above all gods. Well, as those images decking out the standards are ornaments of crosses. All those hangings of your standards and banners are robes of crosses. I praise your zeal: you would not consecrate crosses unclothed and unadorned.
Others, again, certainly with more information and greater verisimilitude, believe that the sun is our god. We shall be counted Persians perhaps, though we do not worship the orb of day painted on a piece of linen cloth, having himself everywhere in his own disk. The idea no doubt has originated from our being known to turn to the east in prayer. But you, many of you, also under pretence sometimes of worshipping the heavenly bodies, move your lips in the direction of the sunrise.
In the same way, if we devote Sun-day to rejoicing, from a far different reason than Sun-worship, we have some resemblance to those of you who devote the day of Saturn to ease and luxury, though they too go far away from Jewish ways, of which indeed they are ignorant. But lately a new edition of our god has been given to the world in that great city: it originated with a certain vile man who was wont to hire himself out to cheat the wild beasts, and who exhibited a picture with this inscription: The God of the Christians, born of an ass. He had the ears of an ass, was hoofed in one foot, carried a book, and wore a toga.
Both the name and the figure gave us amusement. But our opponents ought straightway to have done homage to this biformed divinity, for they have acknowledged gods dog-headed and lion-headed, with horn of buck and ram, with goat-like loins, with serpent legs, with wings sprouting from back or foot. These things we have discussed ex abundanti, that we might not seem willingly to pass by any rumor against us unrefuted. Having thoroughly cleared ourselves, we turn now to an exhibition of what our religion really is.
Source: The Apology (New Advent)