Mark 9:50 Commentary - Whedon's Commentary on the Old and New Testaments

50. Salt is good Whether it be a natural or a spiritual element, it is a good creation of God. As the natural substance is excellent, so it is the rightful symbol of spiritual excellence. If the salt have lost his saltness See notes on Mat 5:13. Have salt in yourselves Let the emblem of active, sharp purity, symbolised by salt, be in you. There is something of sharpness in the reformatory spirit, which is felt to be acrid and unpleasant by those who need the benefit of it. Nevertheless this does not excuse us from the possession of the element. And have peace Let your purity, and your purifying element, with all its sharpness, be characterized by the spirit of love. So in the East salt is the emblem of fidelity and friendship. To eat salt with a man and then be his enemy is the height of treachery.

The same salt which is an element of purity, is also an emblem of peace and perpetuity.