The salt of the earth
Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men. Matthew 5:13
John Stott is correct, I think, when he points out that whereas the Beatitudes teach us about Christian character, these verses teach us about a Christian’s influence in the world. The lesson is straightforward: those who have the character described in the Beatitudes become as an inevitable consequence salt and light in this world. First of all, he tells us that they are salt. Though today we use salt mainly as a flavoring agent, in Jesus’ time salt was also used as a preservative against decay. In an age before refrigeration, if you wanted meat to last, you salted it. If meat is properly cured, it can evidently last a long time. In a similar fashion, the disciples are to act as a moral preservative in a corrupt and godless age. As R. V. G. Tasker put it, they are “called to be a moral disinfectant in a world where moral standards are low, constantly changing, or non-existent . . . they can discharge this function only if they themselves retain their virtue.”
But this is not all Jesus has to say. It is not only, “You are the salt of the world,” but he goes on to say, “But if the salt has lost its savor, wherewith shall it be salted [i.e. how shall it become salty again]? It is thenceforth good for nothing but to be cast out and to be trodden under foot of men.” What is he saying? He is saying that when a Christian ceases to function as salt in the world, they are just useless. A salt without its savor is useful neither for flavoring nor is it useful as a preservative. You just cast it out into the streets.
It should be pointed out that sodium chloride is a stable compound and therefore in the strictest sense salt cannot lose its saltiness. However, the salt that Jesus refers to in this Sermon “derived from salt marshes or the like, rather than by evaporation of salt water, and therefore contained many impurities. The actual salt, being more soluble than the impurities, could be leached out, leaving a residue so dilute it was of little worth.” Both the salt itself and its residue were called salt, so that Jesus could speak of salt that had lost its saltiness. In fact, “in modern Israel savorless salt is still said to be scattered on the soil of flat roofs. This helps harden the soil and prevent leaks, and since the roofs serve as playgrounds and places for public gathering, the salt is still being trodden under foot.”
So this is not just a function of a Christian, it is the function of a Christian in this world. Let it therefore be our aim, not to blend in with the world, but to stand out from the world - although not in a way that is obnoxious but in a way that brings in the good influence of gospel-molded character. May the Lord help us to be the salt of the earth today.
 John Stott, The Message of the Sermon on the Mount (BST), p. 57, ff.
 R. V. G. Tasker, The Gospel According to St. Matthew (TNTC), p. 63.
 D. A. Carson, Matthew: 1-12 (EBC), p. 138.