The salt of the covenant refers to the perpetuity of the covenant God made
Q. What does Mark 9.49 mean where it says that everyone will be salted with fire?
A. To understand this verse we have to put it into context first, and then it will be clearer.
Starting in Mk 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" way at them, tormenting and filling them with anguish.
In v 49, however, the Lord transitions to His own, true believers.
"Everyone" refers to "every follower of mine" and the "fire" with which they must endure refers to "fiery trials" which "season" them (to be salted) 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 the MInchah or bread offering must be salted with salt when on the altar (the fire).
This refers to the fact that the Messiah, that bread from heaven (Jn 6.58), is the one whose sacrifice is for eternity and it cleanses and preserves us to everlasting life (Jn 6.27).
Salt is a preservative, it speaks of eternity and table fellowship. It also was used in cleansing (Ezek 16.4) and also denotes suffering.
Salt also prevents fermentation and it was used on the ramp of the altar in the Temple when there was ice so that the priests would not slip, so it refers to being "sure–footed."
The "salt of the covenant" referred to in Lev 2.13 is referring to the perpetuity of the covenant God made at Mt Sinai, which also included the Brit Chadashah, or "new covenant" (Num 18.19; Deut 29.1 thru 30.11; Jer 31.31,Luke 22.20).
Now, back to our verses in Mark 9.49-50.
Every follower will have fiery trials to go through (1Pet 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 Mk 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 respond 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 to be ready to give a defense to everyone 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 in the thing in which you are slandered (fire), those who revile (the fire) your good behavior (salt) in Christ may be put to shame.
So when it refers to salt being good in Mk 9.50 it is referring to how we react under trials (fire) and the situation is "good" as long as we have left others with a good "taste" in their mouths in that they can’t say we spoke or acted untruthfully or unbecomingly.
In other words, the salt is that preserving, ceaseless, purifying, honest, friendly speech and how we communicate with people and that should be 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 and may come to know the real Lord and Savior.
But if the salt becomes un-flavorful, or does not accomplish the above, what good is it. The verse then concludes with the statement that we should have "salt in yourselves" which means we should retain those valuable and good qualities that will make us a blessing to one another and to all around us and thus, as v 50 says, "be at peace with one another," the result of the flavorful, good salt.