"You are the salt of the earth; but if the salt has become tasteless, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled under foot by men."
This a message to the disciples and how they are different from the world. We are to follow Christ's example. We are to be gentle, hunger and thirst for righteousness, merciful, peacemakers, pure in heart and expect to be persecuted. If we are none of these, if we have become so complacent, then we are of no use. As disciples of Christ, we are the light of the world and we are to share that light with others, but if we are tasteless, how can we do that? We can't! God cannot use us if we are scared, timid, trying to hide our light so others cannot see.
I haven't done a full exegesis of this text or even looked at the Greek yet, but I don't think I am too far off. Feel free to chime in with your own thoughts on this passage.