The argument of Celsus appears to point by these illustrations to this conclusion: that it is “an obligation incumbent on all men to live according to their country's customs, in which case they will escape censure; whereas the Christians, who have abandoned their native usages, and who are not one nation like the Jews, are to be blamed for giving their adherence to the teaching of Jesus.” Let him then tell us whether it is a becoming thing for philosophers, and those who have been taught not to yield to superstition, to abandon their country's customs, so as to eat of those articles of food which are prohibited in their respective cities?
Or whether this proceeding of theirs is opposed to what is becoming? For if, on account of their philosophy, and the instructions which they have received against superstition, they should eat, in disregard of their native laws, what was interdicted by their fathers, why should the Christians (since the Gospel requires them not to busy themselves about statues and images, or even about any of the created works of God but to ascend on high, and present the soul to the Creator); when acting in a similar manner to the philosophers, be censured for so doing?
But if, for the sake of defending the thesis which he has proposed to himself, Celsus, or those who think with him, should say, that even one who had studied philosophy would keep his country's laws, then philosophers in Egypt, for example, would act most ridiculously in avoiding the eating of onions, in order to observe their country's laws, or certain parts of the body, as the head and shoulders, in order not to transgress the traditions of their fathers. And I do not speak of those Egyptians who shudder with fear at the discharge of wind from the body, because if any one of these were to become a philosopher, and still observe the laws of his country, he would be a ridiculous philosopher, acting very unphilosophically. In the same way, then, he who has been led by the Gospel to worship the God of all things, and, from regard to his country's laws, lingers here below among images and statues of men, and does not desire to ascend to the Creator, will resemble those who have indeed learned philosophy, but who are afraid of things which ought to inspire no terrors, and who regard it as an act of impiety to eat of those things which have been enumerated.
Source: Against Celsus (New Advent)