In our last Greek class of the semester, our Prof blew our minds explaining this passage. Now I feel that it is my Christian duty to share this information with you.
First of all, we need to know that the way that this passage is normally translated from the Greek messes with its meaning. A more literal translation of this passage would read:
"You are the salt of the land; but if the salt has been made useless, with what will it [the land] be salted?"
Once we realize that the word "taste" in the English is an interpretive flourish by the translator, it is easier to understand why my Prof made the outlandish claim that this passage is not about table salt, it's about potash.
Potash is also a salt, a different salt than NaCl (table salt), but a salt nonetheless. And the Greek word τὸ ἅλας that we find in Matthew's gospel isn't the word for table salt. It is a generic word for all salt, including salt like potash that is used today as fertilizer (and was used as fertilizer way back in Jesus' day as well).
The more common interpretation of this passage is that the "salt" that Jesus refers to is something that preserves food. So metaphorically, theologians suggest, Christians are what "preserve" the Earth.
But I think my Prof is right when he says Christians are not the "preservers" of the land, we are the "fertilizers" of the land. My Prof might be in the minority of theologians who suggest this, but I have come across others (including a former CMU Prof who blogged about this in 2009). The idea of Christians as a salt fertilizer is consistent with the Christian idea that we lose our identity, we dissolve our ego and become one in Christ so that we can serve God (Matt 10:39).
He made some further claims that I have not been able to verify but he also said that when potash is in the soil, its potassium leaks out, and the potash becomes gypsum, something that is "thrown out and trampled on": further support for the notion that this passage is about a salt fertilizer, not a salt preserver.
So if Jesus says that we are fertilizers of the Earth, what does that mean for us exactly? Unlike the more traditional thinking, we are not meant to purify the Earth, rather, we are meant to scatter and be the agent that allows good things to grow.
I can't help but think about the decline of the Christian church in reference to this passage. Perhaps at the height of Christendom, the land was a little too salty, and it choked out the other plant life. We have biblical references to the practice of over-salting a city to destroy it (Judges 9:45). Soil that is too salty is lifeless. I think the significance of Christians as fertilizers, a significance that has been obscured for too long due to mis-translation, is Good News for Christians today. It seems to be telling us that the decline in Christian churches may not be as worrisome as we think: we are simply returning to the right concentration of Christian fertilizer required to do God's work.