Jesus tells His disciples they are the salt of the earth and the light of the world (Matthew 5:13-16), language drawn from salt’s everyday uses as a preservative, a flavoring, and a valued wage. On its own, though, no one is truly salt of the earth; every attempt at goodness runs into the common misconception that good works alone can preserve or season a life left to itself.
The real answer is imputation: Christ’s righteousness is credited to believers, and it begins producing genuine fruit through the ongoing work of mortification and sanctification. It is the Holy Spirit, not human effort, who supplies the salt that keeps a Christian’s life from losing its savor, so the sermon warns against becoming salt that has lost its saltiness and calls believers back to depend on the Spirit’s ongoing work.