(13) Ye are the salt of the earth.—The words are spoken to the disciples in their ideal character, as the germ of a new Israel, called to a prophetic work, preserving the earth from moral putrescence and decay. The general reference to this antiseptic action of salt is (as in Colossians 4:6, and possibly in the symbolic act of Elisha, 2Kings 2:21) enough to give an adequate meaning to the words, but the special reference to the sacrificial use of salt in Mark 9:49 (see Note there) makes it probable enough that there was some allusion to that thought also here.
If the salt have lost his savour.—The salt commonly used by the Jews of old, as now, came from Jebel-Usdum, on the shores of the Dead Sea, and was known as the Salt of Sodom. Maundrell, the Eastern traveller (circ. A.D. 1690), reports that he found lumps of rock-salt there which had become partially flavourless, but I am not aware that this has been confirmed by recent travellers. Common salt, as is well known, will melt if exposed to moisture, but does not lose its saltness. The question is more curious than important, and does not affect the ideal case represented in our Lord’s words.

Wherewith shall it be salted?—The words imply a relative if not an absolute impossibility. If gifts, graces, blessings, a high calling, and a high work fail, what remains? The parable finds its interpretation in Hebrews 6:1-6.

To be trodden under foot of men.—The Talmud shows (Schottgen in loc.) that the salt which had become unfit for sacrificial use in the store-house was sprinkled in wet weather upon the slopes and steps of the temple to prevent the feet of the priests from slipping, and we may accordingly see in our Lord’s words a possible reference to this practice.
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Ye — Not the apostles, not ministers only; but all who possess and manifest the graces spoken of in the preceding verses, and are truly holy and righteous; are the salt of the earth — Appointed to be the means of preventing or curing the growth of that corruption which prevails in the world, and of seasoning men’s minds with wisdom and grace. But if the salt have lost its savour — Or, be grown insipid, and therefore want seasoning itself, wherewith shall it be salted — By what means can its lost virtue be restored? The word μωρανθη, rendered have lost its savour, has peculiar strength and beauty, and is literally, be infatuated, or, grown foolish, “alluding,” says Dr. Doddridge, “to the common figure, in which sense and spirit are expressed by salt.” It is thenceforth good for nothing — It is wholly useless, and left to be thrown out of doors, and trampled on by men as the common dirt in the streets: “thus worthless and contemptible will you, my disciples, be, even in the most eminent stations, if you lose your character for real and vital religion.” The following passage of Mr. Maundrell, quoted by Dr. Macknight, illustrates our Lord’s supposition of salt’s losing its savour. In the valley of Salt, near Gebul, and about four hours’ journey from Aleppo, there is a small precipice, occasioned by the continual taking away of the salt. “In this,” says he, “you may see how the veins of it lie; I brake a piece off it, of which the part that was exposed to the rain, sun, and air, though it had the sparks and particles of salt, yet it had perfectly lost its savour. The innermost part, which had been connected to the rock, retained its savour, as I found by proof.”
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