Salt of the Earth

The famous passage by Jesus reads: "You are the salt of the earth. But if the salt loses its saltiness, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled by men." (Mt 5:13)
Further references to salt in the Bible (link) would show that salt is considered to be a good thing, as seasoning and covenant (Lev 2:3, Num 18:12) and a bad thing as a herbicide and poison (Judges 9:45, Jer 17:6).
The value or destructiveness of salt has entered even modern English vernacular with such phrases as, "take it with a grain of salt", or "worth your weight in salt", or "pouring salt into a wound".  These expressions have an etymological history behind them.  For instance, Rome's major highway was the Via Salaria, or literally Way of Salt.  The payment for transporting the salt via this road eventually was called "salarium" from which comes the English word, salary, or pay.
There is a superstition that if you spill salt it is bad luck (since it was worth so much, to spill it is a waste & to counter-act the ill fortune, you are to pinch the remaining salt, toss it over your shoulder and let it "fall where it may".  Further about this superstition, we can see in the Leonardo da Vinci painting, The Last Supper, where the artist placed an overturned saltshaker in front of Judas, thus depicting an omen of his ill fate. (see link)
So, while the Bible accurately depicts salt as a valuable thing, some even with which we should season our words, it also depicts it as destructive.  A salt without seasoning properties is good for nothing.  Salty language without purpose, salty doctrine without foundation, salty character without glory to God is good for nothing but to be thrown out into the road & crushed underfoot.
But Christians are supposed to be the good part of salt -- the seasoning of life, where otherwise people's lives are humdrum & really have no meaning, no flavor.  Christians are supposed to be the salt and light of the world.