Leviticus 11:2

Animals which you are to eat, &c. The prohibition of so many kinds of beasts, birds, and fishes, in the law, was ordered, 1. to exercise the people in obedience and temperance; 2. to restrain them from the vices of which these animals were symbols; 3. because the things here forbidden were for the most part unwholesome, and not proper to be eaten; 4. that the people of God, by being obliged to abstain from things corporally unclean, might be trained up to seek a spiritual cleanness. (Challoner)

These animals had no natural uncleanness: for all things are clean to the clean, Titus i. 15. But they were looked upon as such by the prejudice of the people, and many of them possessed noxious qualities. If they had been the most excellent, the will of God is a sufficient reason to enforce the duty of abstinence; (Calmet) as it was in the case of Adam and Eve. As some animals were adored, and others were deemed unclean by the Gentiles, the Hebrews were commanded to sacrifice some of the former description, and to abhor also the latter, that they might never be so foolish, as to imitate the perversity of the nations, in looking upon any animal as a god. (Theod., q. 11.) St. Thomas Aquinas (i. 2. q. 102. a. 6,) explains at large, out of the holy fathers, the different vices, which the unclean animals represent. (Worthington)

By the distinction of these creatures, God would have his people known, chap. xx. 24, 26. Those who chose rather to die than to transgress in this point, are justly honoured by the Church as martyrs, 2 Machabees vi. and vii. (St. Gregory, or. 20.) (Haydock)