matthew 5:13 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. - Free Bible Online

      13-16. We have here the practical application of the foregoing principles to those disciples who sat listening to them, and to their successors in all time. Our Lord, though He began by pronouncing certain characters to be blessed--without express reference to any of His hearers--does not close the beatitudes without intimating that such characters were in existence, and that already they were before Him. Accordingly, from characters He comes to persons possessing them, saying, "Blessed are ye when men shall revile you," &c. (Mt 5:11). And now, continuing this mode of direct personal address, He startles those humble, unknown men by pronouncing them the exalted benefactors of their whole species.
      Ye are the salt of the earth--to preserve it from corruption, to season its insipidity, to freshen and sweeten it. The value of salt for these purposes is abundantly referred to by classical writers as well as in Scripture; and hence its symbolical significance in the religious offerings as well of those without as of those within the pale of revealed religion. In Scripture, mankind, under the unrestrained workings of their own evil nature, are represented as entirely corrupt. Thus, before the flood (Ge 6:11, 12); after the flood (Ge 8:21); in the days of David (Ps 14:2, 3); in the days of Isaiah (Isa 1:5, 6); and in the days of Paul (Eph 2:1-3; see also Job 14:4; 15:15, 16; Joh 3:6; compared with Ro 8:8; Tit 3:2, 3). The remedy for this, says our Lord here, is the active presence of His disciples among their fellows. The character and principles of Christians, brought into close contact with it, are designed to arrest the festering corruption of humanity and season its insipidity. But how, it may be asked, are Christians to do this office for their fellow men, if their righteousness only exasperate them, and recoil, in every form of persecution, upon themselves? The answer is: That is but the first and partial effect of their Christianity upon the world: though the great proportion would dislike and reject the truth, a small but noble band would receive and hold it fast; and in the struggle that would ensue, one and another even of the opposing party would come over to His ranks, and at length the Gospel would carry all before it.
      but if the salt have lost his savour--"become unsavory" or "insipid"; losing its saline or salting property. The meaning is: If that Christianity on which the health of the world depends, does in any age, region, or individual, exist only in name, or if it contain not those saving elements for want of which the world languishes,
      wherewith shall it be salted?--How shall the salting qualities be restored to it? (Compare Mr 9:50). Whether salt ever does lose its saline property--about which there is a difference of opinion--is a question of no moment here. The point of the case lies in the supposition--that if it should lose it, the consequence would be as here described. So with Christians. The question is not: Can, or do, the saints ever totally lose that grace which makes them a blessing to their fellow men? But, What is to be the issue of that Christianity which is found wanting in those elements which can alone stay the corruption and season the tastelessness of an all-pervading carnality? The restoration or non-restoration of grace, or true living Christianity, to those who have lost it, has, in our judgment, nothing at all to do here. The question is not, If a man lose his grace, how shall that grace be restored to him? but, Since living Christianity is the only "salt of the earth," if men lose that, what else can supply its place? What follows is the appalling answer to this question.
      it is thenceforth good for nothing, but to be cast out--a figurative expression of indignant exclusion from the kingdom of God (compare Mt 8:12; 22:13; Joh 6:37; 9:34).
      and to be trodden under foot of men--expressive of contempt and scorn. It is not the mere want of a certain character, but the want of it in those whose profession and appearance were fitted to beget expectation of finding it.

JFB.