But what sort of being is this Ammon of Herodotus, whose words Celsus has quoted, as if by way of demonstrating how each one ought to keep his country's laws? For this Ammon would not allow the people of the cities of Marea and Apis, who inhabit the districts adjacent to Libya, to treat as a matter of indifference the use of cows' flesh, which is a thing not only indifferent in its own nature, but which does not prevent a man from being noble and virtuous. If Ammon, then, forbade the use of cows' flesh, because of the advantage which results from the use of the animal in the cultivation of the ground, and in addition to this, because it is by the female that the breed is increased, the account would possess more plausibility.
But now he simply requires that those who drink of the Nile should observe the laws of the Egyptians regarding cattle. And hereupon Celsus, taking occasion to pass a jest upon the employment of the angels among the Jews as the ambassadors of God, says that “Ammon did not make a worse ambassador of divine things than did the angels of the Jews,” into the meaning of whose words and manifestations he instituted no investigation; otherwise he would have seen, that it is not for oxen that God is concerned, even where He may appear to legislate for them, or for irrational animals, but that what is written for the sake of men, under the appearance of relating to irrational animals, contains certain truths of nature. Celsus, moreover, says that no wrong is committed by any one who wishes to observe the religious worship sanctioned by the laws of his country; and it follows, according to his view, that the Scythians commit no wrong, when, in conformity with their country's laws, they eat human beings.
And those Indians who eat their own fathers are considered, according to Celsus, to do a religious, or at least not a wicked act. He adduces, indeed, a statement of Herodotus which favours the principle that each one ought, from a sense of what is becoming, to obey his country's laws; and he appears to approve of the custom of those Indians called Callatians, who in the time of Darius devoured their parents, since, on Darius inquiring for how great a sum of money they would be willing to lay aside this usage, they raised a loud shout, and bade the king say no more.
Source: Against Celsus (New Advent)