Mark 9:50

Salt is good: but if the salt has lost its saltness, wherewith will ye season it? Have salt in yourselves, and be at peace one with another.

Jesus said of his disciples, "Ye are the salt of the earth," and their saltness would therefore be their quality of having in themselves the likeness and teachings of Jesus. Such salt is indeed good for this world.

Christians are the salt of the earth in the sense of their preserving it from destruction.

If the salt have lost its saltness ... is a metaphor based upon the salt commonly used in Jesus' day, which was not a pure product at all, but mixed with other elements. If the true salt had been leached out, only a worthless residue was left, a perfect metaphor of the Christian who has lost his identity with the Lord.

Have salt in yourselves ... is a reference to the Christian's necessity of keeping his identity with Christ and of continuing faithfully in his teachings.

And be at peace one with another ... is an admonition to brother-love and forbearance, a requirement frequently stressed by the Lord, and absolutely mandatory for all who would follow in his steps.