Proverbs 21:20

So those who hear and out of carelessness forget, so to say swallow what they have heard, so that they no longer have a taste of it in their mouth but just bury what they hear under forgetfulness. But those who meditate on "the law of the Lord day and night" are chewing the cud, as it were, and enjoying the flavor of the word with a kind of palate of the heart.

A desirable treasure, and oil in the dwelling of the just, etc. The cheerfulness of good works, always lovable to God; and the fatness of love shines in the church of the saints. This verse the blessed Pope Gregory in his Homilies on the Gospel placed according to the ancient translation: "A desirable treasure will rest in the mouth of the wise." But the reprobates not only do not attain virtues but also persecute, and as much as they can, hasten to overthrow the good. This verse can certainly be understood about any martyr, in whose dwelling, that is, body, the holy soul, namely the treasure of God, is kept, and the anointing of spiritual grace abounds. But the imprudent persecutor can break such a dwelling; but he cannot touch the treasure and the oil that was held in the dwelling. Hence the Lord teaches not to fear those who kill the body, and after these things have nothing more they can do (Luke 12:4).

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 resteth 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.