13 “You are the salt of the earth. But if salt loses its taste, with what can it be seasoned? It is no longer good for anything but to be thrown out and trampled underfoot. 14 You are the light of the world. A city set on a mountain cannot be hidden. 15 Nor do they light a lamp and then put it under a bushel basket; it is set on a lampstand, where it gives light to all in the house. 16 Just so, your light must shine before others, that they may see your good deeds and glorify your heavenly Father.
As Boring  notes, salt (cf. Mark 9:50; Luke 14:35) had several uses in the ancient world. In the OT, salt was added to sacrifices (Lev 2:13), connected with purity (Ex 30:35; 2 Kgs 2:19–22), symbolic of covenant loyalty (Num 18:19; Ezra 4:14), and used as a seasoning for food (Job 6:6). In the Mishnah salt is associated with wisdom (m. Sotah 9:15). As well, salt was used as a preservative. It is easy to see how all the OT usages of salt would be possible connotations.
The actual Greek is more emphatic than the translation we have in English. The pronoun hymeis (you) is unnecessary. Its presence in the sentence is to give emphasis to what follows. Contextually, it says, “It is you and not those others…” The understanding would not be one of misplaced pride for the hearer, but rather a warning. Salt serves mainly to give flavor, and to prevent corruption. Disciples, if they are true to their calling, make the earth a purer and a more palatable place. But they can do so only as long as they preserve their distinctive character: unsalty salt has no more value.
Jesus was using a proverbial image (it recurs in Bekhoroth 8b). The Rabbis commonly used salt as an image for wisdom (cf. Col. 4:6), which may explain why the Greek word represented by lost its taste actually means ‘become foolish’. (Aramaic tāpēl, which conveys both meanings, was likely the word used by Jesus.) A foolish disciple has no influence on the world. (Boring, 181). To be a disciple and to have no influence in the world is a contradiction.
5:13 loses its taste: mōranthē (from the verb mōraínō) to make foolish
- Eugene Boring, The Gospel of Matthew in The New Interpreter’s Bible, VIII (Nashville, TN: Abingdon Press, 1994) 171-83
- New American Bible available at http://www.usccb.org/bible/books-of-the-bible/index.cfm (2017)