13 You are the salt of the earth. . . .
Chrysostom: When He had delivered to His Apostles such sublime precepts, so much greater than the precepts of the Law, that they might not be dismayed and say, How shall we be able to fulfil these things? He sooths their fears by mingling praises with His instructions, saying, “Ye are the salt of the earth.” This shews them how necessary were these precepts for them. Not for your own salvation merely, or for a single nation, but for the whole world is this doctrine committed to you. It is not for you then to flatter and deal smoothly with men, but, on the contrary, to be rough and biting as salt is. When for thus offending men by reproving them ye are reviled, rejoice; for this is the proper effect of salt to be harsh and grating to the depraved palate. Thus the evil-speaking of others will bring you no inconvenience, but will rather be a testimony of your firmness.
Hilary: The Apostles are preachers of heavenly things, and thus, as it were, salters with eternity; rightly called “the salt of the earth,” as by the virtue of their teaching, they, as it were, salt and preserve bodies for eternity.
Jerome: By the Apostles the whole human race is seasoned.
Remigius: Salt is changed into another kind of substance by three means, water, the heat of the sun, and the breath of the wind. Thus Apostolic men also were changed into spiritual regeneration by the water of baptism, the heat of love, and the breath of the Holy Spirit. That heavenly wisdom also, which the Apostles preached, dries up the humours of carnal works, removes the foulness and putrefaction of evil conversation, kills the work of lustful thoughts, and also that worm of which it is said “their worm dieth not” [Isa 66:24].
In the Old Testament no sacrifice was offered to God unless it were first sprinkled with salt, for none can present an acceptable sacrifice to God without the flavour of heavenly wisdom.
13 . . . But if the salt lose its savour, wherewith shall it be salted? It is good for nothing any more but to be cast out, and to be trodden on by men.
Gloss: When then they who are the heads have fallen away, they are fit for no use but to be cast out from the office of teacher.
Augustine: Not he that suffers persecution is trodden under foot of men, but he who through fear of persecution falls away. For we can tread only on what is below us; but he is no way below us, who however much he may suffer in the body, yet has his heart fixed in heaven.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. I (London: Rivington, 1842).