3 Again, what are we to think of you, or of the better class of Christians among you, some of whom abstain from swine's flesh, some from the flesh of quadrupeds, and some from all animal food, while all the Church admires them for it, and regards them with profound veneration, as only not gods? You obstinately refuse to consider that if the words quoted from the apostle are true and genuine, these people too are misled by doctrines of devils. And there is another observance which no one will venture to explain away or to deny, for it is known to all, and is practised yearly with particular attention in the congregation of Catholics all over the world— I mean the fast of forty days, in the due observance of which a man must abstain from all the things which, according to this verse, were created by God that we might receive them, while at the same time he calls this abstinence a doctrine of devils.
So, my dear friends, shall we say that you too, during this fast, while celebrating the mysteries of Christ's passion, live after the manner of devils, and are deluded by a seducing spirit, and speak lies in hypocrisy, and have your conscience seared with a hot iron? If this does not apply to you, neither does it apply to us. What is to be thought of this verse, or its author; or to whom does it apply, since it agrees neither with the traditions of the Old Testament, nor with the institutions of the New?
As regards the New Testament, the proof is from your own practice; and though the Old requires abstinence only from certain things, still it requires abstinence. On the other hand, this opinion of yours makes all abstinence from animal food a doctrine of devils. If this is your belief, once more I say it, you must condemn Moses, and reject the prophets, and pass the same sentence on yourselves; for, as they always abstained from certain kinds of food, so you sometimes abstain from all food.
Source: Reply to Faustus the Manichaean (New Advent)