Mar 9:49 For every one shall be salted with fire, and every sacrifice shall be salted with salt.
Alternative: For all shall be gathered in fire and all sacrifices shall be gathered in salt.
Alternative: For all shall be preserved in fire and all sacrifices shall be preserved in salt.
Before reading this post, you may want to read this earlier post on the meaning of salt in Christ's time.
Concluding as it does this chapter on the temporal and the spiritual world, this verse brings together a lot of threads into a very special view of the nature of the universe. In the verses previous to this, Christ has been talking about wasting our lives, literally, putting them on the trash heap where they rot, getting filled with worms, and burn. The worm of self can only die in this world because the fires can only destroy it in this world. But but in the eternal world of spirit, everything is preserved both good (sacrifices) and bad (rotten).
Continuing this discussion about the nature of fire, Christ here draws the parallel between the purifying fires of Gehenna and the fires of sacrifice. The difference is that sacrifices are still valuable when they are burnt while the rubbish of Gehenna is rotten, filled with worms.
In our world, fire is the gateway to the eternal because it removes things from this world. Fire erases its physical form. The spirit is unchanged, even the spirit of the first itself. Fire consume material things in the same way that our temporal life is consumed by time.
Spiritual fire doesn't destroy. The spiritual world is eternal and unchanging. It is the concept of light and heat and change but without the ability to change.
Christ uses the symbol of fire in Mat 13:30 to represent the baking of bread in a very subtle way. In that parable, weed and grain are allowed to grow together so that at harvest time, the weeds can provide fuel to bake the bread. This is another way of expressing the productive value of fire. In that version, the weeds instead of destroying the grain are actually useful in transforming them. Without the choice evil in the world, we could reach our spiritual potential.
"Shall be salted" is from the word halizô or halizô (halizo). Its first meaning (the first link) is "to gather together," "assemble' (of military forces), "collect" of fragments, and "amassing into a globe." Its second meaning is "salt," "to be salted," "to provide salt," and "to salt food."
"Fire" is from pur (pyr), which means "fire," "sacrificial fire," "funeral fire," "hearth-fire," "lightning," "the light of torches," and "heat of fever."
"Sacrifice" is thusia (thysia), which means "a burnt-offering," "a sacrifice," "a victim of sacrifice," "mode of sacrifice," "festival at which sacrifices are offered," "rite," and "ceremony."
"Salt" is from hals (hals), which means "salt," "salt-rock," "brine," and is a metaphor for "sales" and "wit."