Any one, indeed, who chooses, may relate how the various quarters of the earth, being distributed among certain governing powers, are administered by those who superintend them; but let him tell us also how what is done among each nation is done rightly when agreeable to the wishes of the superintendents. Let him, for example, tell us whether the laws of the Scythians, which permit the murder of parents, are right laws; or those of the Persians, which do not forbid the marriages of sons with their mothers, or of daughters with their own fathers.
But what need is there for me to make selections from those who have been engaged in the business of enacting laws among the different nations, and to inquire how the laws are rightly enacted among each, according as they please the superintending powers? Let Celsus, however, tell us how it would be an act of impiety to get rid of those ancestral laws which permit the marriages of mothers and daughters; or which pronounce a man happy who puts an end to his life by hanging, or declare that they undergo entire purification who deliver themselves over to the fire, and who terminate their existence by fire; and how it is an act of impiety to do away with those laws which, for example, prevail in the Tauric Chersonese, regarding the offering up of strangers in sacrifice to Diana, or among certain of the Libyan tribes regarding the sacrifice of children to Saturn.
Moreover, this inference follows from the dictum of Celsus, that it is an act of impiety on the part of the Jews to do away with those ancestral laws which forbid the worship of any other deity than the Creator of all things. And it will follow, according to his view, that piety is not divine by its own nature, but by a certain (external) arrangement and appointment. For it is an act of piety among certain tribes to worship a crocodile, and to eat what is an object of adoration among other tribes; while, again, with others it is a pious act to worship a calf, and among others, again, to regard the goat as a god.
And, in this way, the same individual will be regarded as acting piously according to one set of laws, and impiously according to another; and this is the most absurd result that can be conceived!
Source: Against Celsus (New Advent)