Salt was precious in the ancient world — it preserved food from decay, gave flavour, and sealed covenants (the Bible speaks of a covenant of salt, an unbreakable bond). It was even rubbed on newborns and offered with sacrifices. To be salt was to be a preserving, purifying, life-giving presence.
Jesus told his followers, ye are the salt of the earth — but if the salt have lost his savour... it is thenceforth good for nothing. Christians are meant to be a preserving influence in a decaying world, slowing corruption and bringing out goodness, often quietly and unseen. But salt that loses its distinctiveness is useless. The call is to stay salty — genuinely different, genuinely good, genuinely present in the world.
Salt in Scripture
Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men.