“Salt, city, and light can be used for almost anything, and the history of interpretation shows that this indeed is what has happened” (Luz, Matthew 1-7, 205).
The second section of the Sermon makes two remarkable statements about the followers of Jesus. They are the “salt of the Earth” and the “light of the world.” Both metaphors have become common in western culture, although the meaning of “salt of the earth” has changed. For example, the common dictionary definition is “a simple, good person” just as Jesus’s followers were simple fishermen. But this completely misses the point of what Jesus intended in Matthew 5:13.
So what does Jesus mean by these two metaphors? As is common in the teaching of Jesus, he is looking back to the Hebrew Bible and interpreting as a prophet by applying texts and metaphors to himself and his followers.
Since salt is a preserving agent in the ancient world, the followers of Jesus will in some real way act as agents of preservation. Salt has several different uses, from purification (Exod 30:35) to adding flavor to foods (Luke 14:34, “lost its taste”). Scot McKnight suggests the exact nuance of “salt” is less important than the loss of saltiness (McKnight, Sermon on the Mount, 57; Davies and Allison list eleven possibilities, Matthew 1-7, 472-3). Whatever the use of salt Jesus as in mind, salt is worthless if it is not salty! The verb μωραίνω sometimes has the nuance of “foolish” here the aorist passive refers to something which has become tasteless, or possibly “become insipid.”
Can salt actually lose its saltiness? It seems unlikely for the chemistry of salt to change into something else simply through disuse, so scholars often refer to Dead Sea salt, which is only about one-third salt. The other minerals can lose their taste when they dry out. It is possible the reference is mixing salt and other things, so that the salt is no longer effective.
Or, is “salt losing its saltiness” a metaphor for an impossible thing, like hiding a city on the hill? If it is impossible for a city on a hill to be hidden, perhaps the point is that in the unlikely event salt goes bad, it gets tossed out. This may be an eschatological allusion, tramping under foot may be part of judgment.
“The salt is thrown out, according to the everything-in-the-street law, which was the principle of garbage disposal in the ancient Orient. Understand that the disciple will be excluded from Jesus’ following … will be trodden underfoot; an image of the scorn—even on the part of humans—that is the lot of disciples who have fallen away from their fervor” (M. J. Lagrange, cited by TLNT 2:536).
This metaphor implies the follower of Jesus can become less effective, so that they are “worthless.” Looking ahead to the end of the Sermon, Jesus says many will come to him on the Day of Judgment expecting to enter into the Kingdom of God, claiming to have prophesied and cast out demons in his name, but he will say to them “I never knew you” (Matt 7:21-23). Not everyone who appears to be a follower of Christ is actually a follower, just as not everyone in a church today has a real relationship with Jesus.
This saying is spoken directly to Jesus’s followers, the ones who are sitting at his feet and listening to his teaching. They are the ones who are told they are a preserving agent designed to keep their culture from decaying into foolishness. It is perhaps not insignificant the word Jesus uses is also used by Paul in Romans 1:22, those who claimed to me wise had become fools when they worshiped idols. The follower of Jesus potentially can decay from a wise person (with their house built on the rock) into the foolish person (with their house built on the sand), as Jesus will conclude the sermon in Matthew 7:24-26.
This is not particularly comforting. Jesus says it is possible for his followers to become “worthless” and no longer of any value. On the one hand, this may be part of a common theme throughout Matthew that there are some followers of Jesus who are not “true followers” and will be separated out for judgment at some point (Judas, for example). But on the other hand, this is a warning to all the followers of Jesus to maintain their effectiveness as disciples of Jesus.
In what ways might the church (or an individual Christian) “lose their saltiness”? Is it possible some parts of the western, Christian church has already become ineffective for the Gospel and has become worthless? What are some ways the Church heed this warning?