For that which is offered to idols is sacrificed to demons, and a man of God must not join the table of demons. As to things strangled, we are forbidden by Scripture to partake of them, because the blood is still in them; and blood, especially the odour arising from blood, is said to be the food of demons. Perhaps, then, if we were to eat of strangled animals, we might have such spirits feeding along with us. And the reason which forbids the use of strangled animals for food is also applicable to the use of blood.
And it may not be amiss, as bearing on this point, to recall a beautiful saying in the writings of Sextus, which is known to most Christians: “The eating of animals,” says he, “is a matter of indifference; but to abstain from them is more agreeable to reason.” It is not, therefore, simply an account of some traditions of our fathers that we refrain from eating victims offered to those called gods or heroes or demons, but for other reasons, some of which I have here mentioned.
It is not to be supposed, however, that we are to abstain from the flesh of animals in the same way as we are bound to abstain from all race and wickedness: we are indeed to abstain not only from the flesh of animals, but from all other kinds of food, if we cannot partake of them without incurring evil, and the consequences of evil. For we are to avoid eating for gluttony, or for the mere gratification of the appetite, without regard to the health and sustenance of the body. We do not believe that souls pass from one body to another, and that they may descend so low as to enter the bodies of the brutes.
If we abstain at times from eating the flesh of animals, it is evidently, therefore, not for the same reason as Pythagoras; for it is the reasonable soul alone that we honour, and we commit its bodily organs with due honours to the grave. For it is not right that the dwelling-place of the rational soul should be cast aside anywhere without honour, like the carcasses of brute beasts; and so much the more when we believe that the respect paid to the body redounds to the honour of the person who received from God a soul which has nobly employed the organs of the body in which it resided. In regard to the question, “How are the dead raised up, and with what body do they come?” we have already answered it briefly, as our purpose required.
Source: Against Celsus (New Advent)