To understand this verse, we must put it into a proper context first. Starting in Mark 9.39-48, the Lord is talking about what will happen to those who reject him. They will go to the Lake of fire (Rev 20.10) “where their worm does not die, and the fire is not quenched” quoting Isa 66.24. Their “worm” is a Hebrew idiom meaning their “conscience” and how it will “gnaw” away at them, tormenting them with anguish.
In Mark 9.49, however, the Lord transitions to his own followers, the true believers. “Everyone” refers to the followers of Yeshua and the “fire” refers to the “fiery trials” which we go through which “season” (to be salted) us because the “servant is not above his master” and the Lord went through the very same thing. This salting refers back to Lev 2.13 where it says that the Minchah, or bread offering, must be salted with salt when put on the altar (the fire). This refers to the fact that the Messiah, that bread from heaven (John 6.58), is the one whose sacrifice is for eternity and it cleanses and preserves us to everlasting life (John 6.27).
Salt is a preservative and it speaks of eternity and table fellowship among friends. It also was used for cleansing (Ezek 16.4) and it denotes suffering. Salt also prevents fermentation and it was used on the ramp leading up to the altar in the Temple when it was icy so that the priests would not slip, resulting in being “sure footed.” The “salt of the covenant” referred to in Lev 2.13 is referring to the perpetuity of the covenant God made with Israel at Mt Sinai (Num 18.19; Deut 29.1 through 30.11, including the Brit Chadasha, or “renewed” covenant in Jer 31.31; Luke 22.20). Now, back to our verses in Mark 9.49-50.
Every believer will have “fiery trials” to go through (1 Pet 1.7) and to be an acceptable sacrifice and well pleasing to the Lord, we must be salted. Matt 5.13 says that we are the “salt of the earth” and Mark 9.50 says that “salt is good.” So, what is this salt? In Col 4.6 it says that our speech should be “salted with salt” so that we may know how to sincerely reply to each person. In 1 Pet 3.15-16 it goes a little deeper in that we are to sanctify, or set apart, the Lord in our hearts and be ready to give a defense to anyone who asks us to give an account for the hope that is within us, yet with gentleness and reverence (salt), and to keep a good conscience (salt) so that the thing in which we are slandered (the fire), those who revile (fire) your good behavior (salt) in Messiah may be put to shame.
So when it refers to “salt” being good in Mark 9.50 it is referring to how we react under trials (the fire) and the situation is “good” as long as we have left others with a good “taste” in their mouths so that they can’t say we spoke or acted untruthfully or unbecomingly. In other words, the salt is that which preserves, it is ceaseless, purifying, honest and friendly in regards to our speech and how we communicate with people. It is a positive witness and example to an unbeliever.
This “salt” will help remove the “fermentation” of sin in their lives and through our words they can have an accurate picture of the Lord and come to know the real Lord and savior. But if the salt has lost its flavor, or does not accomplish the above, what good is it. Then v 50 concludes with the statement that we should have “salt within yourselves” which means that we should retain in ourselves those valuable and good qualities that will make us a blessing to one another. The end result will be that we are at “peace with one another” which is the desired result of good, flavorful salt.