Matthew 5:13

Christ had lately called his disciples, and told them that they should be fishers of men; here he tells them further what he designed them to be - the salt of the earth, and lights of the world, that they might be indeed what it was expected they should be. I. Ye are the salt of the earth. This would encourage and support them under their sufferings, that, though they should be treated with contempt, yet they should really be blessings to the world, and the more so for their suffering thus. The prophets, who went before them, were the salt of the land of Canaan; but the apostles were the salt of the whole earth, for they must go into all the world to preach the gospel. It was a discouragement to them that they were so few and so weak. What could they do in so large a province as the whole earth? Nothing, if they were to work by force of arms and dint of sword; but, being to work silent as salt, one handful of that salt would diffuse its savour far and wide; would go a great way, and work insensibly and irresistibly as leaven, Mat 13:33. The doctrine of the gospel is as salt; it is penetrating, quick, and powerful (Heb 4:12); it reaches the heart Act 2:37. It is cleansing, it is relishing, and preserves from putrefaction.

— Matthew Henry's Concise Commentary (public domain)

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. (Mat 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 (Gen 6:11-12); after the flood (Gen 8:21); in the days of David (Psa 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; Job 15:15-16; Joh 3:6; compared with Rom 8:8; Tit 3:2-3).

— Jamieson-Fausset-Brown Commentary (public domain)