7 The saying of the apostle, that "to the pure all things are pure," and that "every creature of God is good," is not opposed to the prohibitions of the Old Testament; and the explanation, if they can understand it, is this. The apostle speaks of the natures of the things, while the Old Testament calls some animals unclean, not in their nature, but symbolically, on account of the prefigurative character of that dispensation. For instance, a pig and a lamb are both clean in their nature, for every creature of God is good; but symbolically, a lamb is clean, and a pig unclean.
So the words wise and fool are both clean in their nature, as words composed of letters but fool may be called symbolically unclean, because it means an unclean thing. Perhaps a pig is the same among symbols as a fool is among real things. The animal, and the four letters which compose the word, may mean the same thing. No doubt the animal is pronounced unclean by the law, because it does not chew the cud; which is not a fault but its nature. But the men of whom this animal is a symbol are unclean, not by nature, but from their own fault; because, though they gladly hear the words of wisdom, they never reflect on them afterwards.
For to recall, in quiet repose, some useful instruction from the stomach of memory to the mouth of reflection, is a kind of spiritual rumination. The animals above mentioned are a symbol of those people who do not do this. And the prohibition of the flesh of these animals is a warning against this fault. Another passage of Scripture speaks of the precious treasure of wisdom, and describes ruminating as clean, and not ruminating as unclean: "A precious treasure rests in the mouth of a wise man; but a foolish man swallows it up." Symbols of this kind, either in words or in things, give useful and pleasant exercise to intelligent minds in the way of inquiry and comparison.
But formerly people were required not only to hear, but to practise many such things. For at that time it was necessary that, by deeds as well as by words, those things should be foreshadowed which were in after times to be revealed. After the revelation by Christ and in Christ, the community of believers is not burdened with the practice of the observances, but is admonished to give heed to the prophecy. This is our reason for accounting no animals unclean, in accordance with the saying of the Lord and of the apostle, while we are not opposed to the Old Testament, where some animals are pronounced unclean. Now let us hear why you consider all animal food unclean.
Source: Reply to Faustus the Manichaean (New Advent)