“if the salt has become unsalty.” For how salt can become unsalty, see commentary on Matthew 5:13.
“how can it be made salty again?” To understand why Jesus would say this, we have to remember the context of this statement. Jesus has just finished talking about how we must remove causes to sin because the alternative is destruction in the fire of Gehenna (cp. Mark 9:43, 45, 47). So, when Jesus says, after salt has become unsalty, “How can you make it salty again,” the implied answer is that you cannot. He goes on to say in the parallel in Matthew 5:13 that it is not useful for anything but to be trampled under people’s feet. He says this because that is the true reality of salt in the real world and it helps communicate a thought that Jesus wanted to communicate about not being able to be restored from the fire of Gehenna.
In biblical times, and still to an extent in modern times, a major source of salt was the marshes along the seashore where evaporated salt water left salt deposits. The salt, which was not pure, was sometimes stored in houses or left on the ground, and in both cases, it can become ruined, thus “unsalty.” William Thomson details a story eerily similar to what we find Jesus describing here in Mark 9:50. “These houses have merely earthen floors [i.e., have only dirt floors], and the salt next [to] the ground in a few years [is] entirely spoiled. I saw large quantities of it literally thrown into the street, to be trodden underfoot of men and beasts.”a This thought is parallel to the idea of Gehenna; no one can be restored from the fire of Gehenna, it is the everlasting destruction of the unbeliever (2 Thess. 1:9), the unsalty.
Here is the meaning of the metaphor: Everyone will be seasoned with fire (Mark 9:49), some people will be salty (believers) and will be a pleasing sacrifice to God (Lev. 2:13). They have salt in themselves (Mark 9:50). The rest will also be seasoned with fire, yet they are not “salty” because their salt has lost its taste, they will not be a pleasing sacrifice to God (Lev. 2:13) and will be destroyed in the fire of Gehenna (Mark 9:43, 45).
[For more on how salt can become “unsalty,” see commentary on Matt. 5:13. For more on annihilation in Gehenna, the Lake of Fire, see Appendix 5, “Annihilation in the Lake of Fire.”]
“have salt in yourselves.” This obscure saying is connected to both the salt covenant and to the fact that every burnt sacrifice that was offered to God was salted with salt. The salt covenant was a way to make a binding agreement between people. The salt covenant is sometimes known as the “friendship covenant” because even when enemies ate salt together they were bound by the covenant to help and protect each other. It was not always possible to eat a meal together and so have “peace” (the Hebrew is “shalom” and means more than just “peace,” it means well-being), but if people had salt in themselves they would have shalom with those they met. Followers of Jesus are, like him, to be as much at peace with the world around them as it is possible to be (cp. Rom. 12:18).
[For more on the salt covenant, see commentary on 2 Chron. 13:5. For more on sacrifices and offerings being offered with salt, see Lev. 2:13; Num. 18:17-19; and see commentary on Lev. 2:13.]
“one another.” The phrase “one another” occurs in the context of the community of believers, and while we are to be good to everyone, in the context of the New Testament Epistles, the commands toward “one another” are specifically to other believers. For example, Christians are to be “especially good to the household of faith” (Gal. 6:10). It is very important for the richness of our lives together here on earth, for our personal growth here on earth, and for rewards in the next life, that each Christian needs to be “other focused;” focused on others and how we can help them. The phrase “one another” occurs many times in the New Testament, stating and reinforcing that truth.
[For more on the “one another” commands, see commentary on Galatians 5:13, “one another.” For more on “love one another,” see commentary on John 13:34.]