Salt of the earth” is one of those canned phrases that we chuck around without really thinking about it. It’s usually a compliment. But have you ever actually tried putting salt on the earth? It’s a great way to completely screw it up. Nothing grows if you put salt on the earth. So describing someone as “the salt of the earth” is like saying “Wherever they go, they create a barren, poisoned wasteland”.
The phrase, like many other good soundbites, originates with Jesus. But what did he actually mean by saying in the Sermon on the Mount (Matthew 5:13), “Ye [disciples] are the salt of the earth”? Incidentally, he also told them, “Ye are the light of the world,” which makes an odd contrast with John 8:12 (“I am the light of the world.” Which is it Jesus, make up your mind.)
First of all, in New Testament times salt was very valuable. It is a terrific preservative, and makes things taste great. Just think of bacon! Whether or not salt was worth its weight in gold, it was economically pretty important for most of human history.
One of those things that everyone thinks they know is that Roman soldiers were paid in salt (hence the term ‘salary’). This is not true, or at least I don’t know of any evidence for it. If you think about it how would that actually work? You bring a wheelbarrow to work on payday and take it home piled high with salt? Wouldn’t it be more useful if they paid you salt one week, then some fish and chips the week after, so that you didn’t end up with just a house full of salt?
There are those who argue that Jesus was using the metaphor of salt in its preservative sense. The disciples are charged with preserving the Earth, being the guardians of the world and keeping it pure. This is not very convincing. Jesus was an apocalyptic prophet who believed that the world as we know it would shortly come to an end (“The time is fulfilled, and the kingdom of God is at hand” - Mark 1:15), so why would he charge the disciples to preserve it?
Presumably Jesus did not mean the phrase as an insult, so what did he mean? Another suggestion is that he meant it in the sense of salt being a fertilizer (which it is, in small quantities). The disciples should nourish and fertilize the seeds of Jesus’s message (and Jesus was keen on seed and planting metaphors).
This seems more plausible, but there is a problem. Salt is only good for the earth in tiny quantities. Too much kills the plants and renders the soil useless. So was Jesus saying that there should only be a small number of disciples, and that a large following would poison his message and damage the world? Not likely. Alan Kreider explores the salt metaphor in an essay called Salty Discipleship and quotes Paul Minear as saying “The extreme variety of interpretations… indicates the absence of any decisive clue to its original meaning.”
One further suggestion is that Jesus was referencing the fact that salt was a symbol of God’s covenant with his people, thanks to its being a mandatory addition to animal sacrifices (“And every oblation of thy meat offering shalt thou season with salt; neither shalt thou suffer the salt of the covenant of thy God to be lacking from thy meat offering: with all thine offerings thou shalt offer salt” - Lev 2:13).
Jesus was a great one for quoting Scripture. He seems to have an Old Testament quote for every occasion, though he frequently adds to or changes the message of the passages he quotes (“Ye have heard that it hath been said, An eye for an eye, and a tooth for a tooth [Lev 24:19-20] But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.” - Matthew 5:38-39)
So it seems at least arguable that Jesus meant to say that the disciples were like salt in the sense that they were an essential part of his sacrifice. Jesus, after all, is the “Lamb of God” (John 1:29) and says in Mark 9:49 that “Every one shall be salted [preserved] with fire, and every sacrifice shall be salted with salt.”
Alternatively, maybe Jesus just really liked salt.