It is an interesting juxtaposition to read Leviticus and Mark 9:42-49 on the same day. Leviticus details God's covenantal requirements and the punishments for breaking them, which range from offering a sacrifice of an animal to the sacrifice of your life.
In Mark 9, Jesus said you'd be better off dead by drowning (a horrible death for a desert person to consider) than to lead someone in to sin, and better off cutting off a body part than sinning. God takes sin very seriously.
Verse 49 of Mark 9 appears to be an allusion to Leviticus also. You had to offer salt with any sacrifice. We offer ourselves as a sacrifice to God, salted with discipline and suffering (fire), and come out still salted, or preserved and purified. In contrast, the pretender falls away in suffering. His saltiness is lost and cannot be restored.