Adam Clarke's Commentary on the Bible

Verse 13 (Matthew 5:13)

Ye are the salt of the earth - Our Lord shows here what the preachers of the Gospel, and what all who profess to follow him, should be; the salt of the earth, to preserve the world from putrefaction and destruction. See the note on Leviticus 2:13 .

But if the salt have lost his savor - That this is possible in the land of Judea, we have proof from Mr. Maundrell, who, describing the Valley of Salt, speaks thus: "Along, on one side of the valley, toward Gibul, there is a small precipice about two men's lengths, occasioned by the continual taking away of the salt; and, in this, you may see how the veins of it lie. I broke a piece of it, of which that 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 inner part, which was connected to the rock, retained its savor, as I found by proof." See his Trav., 5th edit., last page. A preacher, or private Christian, who has lost the life of Christ, and the witness of his Spirit, out of his soul, may be likened to this salt. He may have the sparks and glittering particles of true wisdom, but without its unction or comfort. Only that which is connected with the rock, the soul that is in union with Christ Jesus by the Holy Spirit, can preserve its savor, and be instrumental of good to others.

To be trodden underfoot - There was a species of salt in Judea, which was generated at the lake Asphaltites, and hence called bituminous salt, easily rendered vapid, and of no other use but to be spread in a part of the temple, to prevent slipping in wet weather. This is probably what our Lord alludes to in this place. The existence of such a salt, and its application to such a use, Schoettgenius has largely proved in his Horae Hebraicae, vol. i. p. 18, etc.

- Adam Clarke's Commentary on the Bible