Jesus instructs his disciples to “be at peace with one another” (Mark 9:50, ESV). It appears as an odd place to add the instruction to be at peace with one another, with the warning about purification from sin, but it is not as odd as we think. Salt, in this metaphor, symbolized the spiritual cleansing (or purification) of the disciple.
This purification is the dominant trait of a disciple of Christ. Jesus emphasized the need for this spiritual purification because he was concerned for the world and wanted to reach the world for the Kingdom. The disciple of Christ was the vehicle God was going to use to win the world for Christ.
Disciples have a responsibility to Christ and the Kingdom to not lose this “saltiness” quality. Jesus’ command to “be at peace with one another” is consistent with this quality of the disciple. What sets us as disciples apart from the world is this “saltiness.” Strife and discord are inconsistent with this quality of saltiness, and the instruction from Jesus is to not lose this purification.
Jesus was also addressing the contentious attitude of the disciples as to who was going to be the greatest among them (Mark 9:33-34, ESV). Jesus was warning His disciples of the potential to lose their saltiness and their witness in the world if strife and factions develop.