But neither do the Jews pride themselves upon abstaining from swine's flesh, as if it were some great thing; but upon their having ascertained the nature of clean and unclean animals, and the cause of the distinction, and of swine being classed among the unclean. And these distinctions were signs of certain things until the advent of Jesus; after whose coming it was said to His disciple, who did not yet comprehend the doctrine concerning these matters, but who said, “Nothing that is common or unclean has entered into my mouth,” “What God has cleansed, call not common.” It therefore in no way affects either the Jews or us that the Egyptian priests abstain not only from the flesh of swine, but also from that of goats, and sheep, and oxen, and fish. But since it is not that “which enters into the mouth that defiles a man,” and since “meat does not commend us to God,” we do not set great store on refraining from eating, nor yet are we induced to eat from a gluttonous appetite. And therefore, so far as we are concerned, the followers of Pythagoras, who abstain from all things that contain life may do as they please; only observe the different reason for abstaining from things that have life on the part of the Pythagoreans and our ascetics. For the former abstain on account of the fable about the transmigration of souls, as the poet says:—
And some one, lifting up his beloved son,
Will slay him after prayer; O how foolish he!
We, however, when we do abstain, do so because “we keep under our body, and bring it into subjection,” and desire “to mortify our members that are upon the earth, fornication, uncleanness, inordinate affection, evil concupiscence;” and we use every effort to “mortify the deeds of the flesh.”
Source: Against Celsus (New Advent)