“if the salt has become unsalty.” For how salt can become unsalty, see commentary on Matthew 5:13.
“have salt in yourselves.” This obscure saying is connected to the salt covenant, which was a way to make a binding agreement between people. The salt covenant is sometimes called 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].
“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].