Jesus asked this to show how serious it would be for His followers to lose what makes them useful. Salt is valued for its saltiness. If it loses that, nothing else can put it back. Barnes explains that salt keeps things from spoiling, and that Christians, "by their lives and instructions, are to keep the world from entire moral corruption." Matthew Henry similarly says Christ sent His disciples to "season it with knowledge and grace." The question invites the hearers to supply the obvious answer themselves: there is nothing left to salt the salt.

Jesus answers His own question in the same verse: "it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men" (Matthew 5:13). He then gives a second picture of the same calling: "Ye are the light of the world" (Matthew 5:14). He ends the thought with a command: "Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven" (Matthew 5:16).

Barnes explains that salt in that region was often impure, mixed with earth and other substances. When exposed to rain and sun, the true salt could wash out, leaving a residue that looked like salt but had no saltiness. Barnes then quotes a traveler's report that such salt "actually destroys all fertility wherever it is thrown; and this is the reason why it is cast into the street." So it could not even be used on a field.

The Greek verb for "have lost his savour" is moranthe, from moraino. Thayer says that in the Bible it can mean to make foolish, or, of salt, to make flat and tasteless. The same verb is used in Romans 1:22 of people who "became fools."

Jesus does not tell His disciples to become salt. He says they already are: "Ye are the salt of the earth." Their worth comes from His calling. The question then guards that calling. A disciple is useful to the world only while he remains what Jesus made him.

Commentators apply the warning in different ways. Gill holds that the savour that can be lost is not true grace, "which cannot be lost," but something else, such as gifts for the ministry, sound doctrine, or an outward show of godliness. Matthew Henry applies it to anyone who takes up the name of Christ without grace: "If a man can take up the profession of Christ, and yet remain graceless, no other doctrine, no other means, can make him profitable." Both agree on the point Jesus presses: a disciple who loses what makes him distinct has nothing left to offer the world.