Jesus uses salt to show what His disciples are meant to be, and what is lost if they lose it. The question expects the answer "nothing": salt that has lost its saltness cannot be salted again. Barnes explains "have salt in yourselves" as keeping "the principles of denying yourselves, of suppressing pride, ambition, contention." That ties the saying back to how the conversation began, with the twelve arguing about who was greatest (Mark 9:34). Gill sees salt as the grace of God, which seasons the heart and preserves "from the corruption of sin."

The question answers itself: once salt loses its saltness, nothing can restore it. Jesus does not leave the disciples with only a warning. He turns it into a command: "Have salt in yourselves, and have peace one with another" (Mark 9:50). In Luke's form of the saying, such salt "is neither fit for the land, nor yet for the dunghill; but men cast it out" (Luke 14:35).

Salt was part of Israel's worship. The Law said, "with all thine offerings thou shalt offer salt" (Leviticus 2:13), a law Gill brings in at Mark 9:49. Jesus has just said that "every sacrifice shall be salted with salt" (Mark 9:49).

The Greek word for salt that has "lost his saltness" is analos, which Thayer defines as "saltless, unsalted"; it appears only here in the New Testament. How could salt lose its taste? Barnes explains (at Matthew 5:13) that the salt used in that region was impure, "mingled with vegetable and earthy substances," so that it could lose its saltness and leave only earthy matter behind. Such salt, Jesus says, is "good for nothing, but to be cast out, and to be trodden under foot of men" (Matthew 5:13).

Jesus asks this question to remind His followers that they are meant to make a difference, and that this is no small thing to lose. Matthew Henry puts it plainly: "if this salt lose its saltiness, if a Christian revolt from his Christianity... what can recover him, or wherewith will ye season him?"

The verse before this one is hard. Barnes says, "Perhaps no passage in the New Testament has given more perplexity to commentators than this." He adds that it "may be impossible now to fix its precise meaning," and suggests that being "salted with fire" probably means the self-denials and trials that make disciples acceptable offerings to God. Gill reads the fire mainly as judgment, though he also allows that believers are seasoned by grace. Either way, Jesus' closing words point to what He wants: "Have salt in yourselves, and have peace one with another" (Mark 9:50). A conversation that began with rivalry (Mark 9:34) ends with a call to peace.