John Calvin Commentary Mark 9:50

"Salt is good: but if the salt have lost its saltness, wherewith will ye season it? Have salt in yourselves, and be at peace one with another." — Mark 9:50 (ASV)

Have salt in yourselves. This phrase may be understood in a different sense from its meaning in the previous verse. It can signify that seasoning of good fragrance which is obtained by faith, or rather, the wisdom of the Spirit.

When Paul instructs, Let your speech be always with grace, seasoned with salt, (Colossians 4:6), he means that we ought to be holy, purified from all profane follies and corruptions, and filled with spiritual grace. This grace edifies all who hear it and spreads its sweet fragrance over them.

If this interpretation is adopted, it may be necessary to understand the latter clause as referring to the mutual peace promoted by that salt.

However, since it is more probable that this last sentence depends on the preceding discourse, I believe Christ is exhorting his own people to maintain the integrity of faith, which can also serve to purify others: “You must make every effort, not only to be salted within, but also to salt others.”

But as salt bites with its sharpness, he immediately admonishes them to regulate this seasoning in such a way that peace may be fully preserved with one another.