What does Mark 9:50 mean?

Salt, of course, cannot lose its saltiness, but the purity of salt varies. Salt harvested from the Dead Sea, in particular, grows increasingly stale because of those impurities. If the salt is mixed with too many other minerals, the taste will become so diluted it will be useless. The disciples risk this when they allow the values of the world, like love of authority and power and position, to suppress the purified saltiness that God desires.

Another way to interpret this remark is to note that "saltiness" is the purpose of salt; it's a defining characteristic of that mineral. If something so crucial to its identity can be lost, how could it ever be restored? This suggests a need for believers to treat their role in the kingdom of God as crucial, not secondary.

In a similar way, in Matthew 5:13, Jesus calls us to be the salt of the earth. We are to be His agents in purifying the world. Like salt, we are to bring out the better nature of those around us. The result will naturally be greater peace with each other. The sentiment brings to mind the ancient salt vow. If two people shared salt, they were bound to defend and support each other, even if they had been enemies. The disciples should live out a similar vow with each other.

Jesus' last phrase brings us back around to Mark 9:33–35 when the disciples are arguing about who is greatest. While they are worried about their position in Jesus' kingdom, He is concerned with the church reaching new believers (Mark 9:36–37, 39–40) and the personal sacrifices required for obedience (Mark 9:42–47).

Matthew and Luke add that the un-salty will be thrown out and trampled on, which is basically what the disciples wanted to do to the man who was exorcising demons in Jesus' name (Mark 9:38). Jesus explains that position and authority are not necessarily markers of how closely we follow Him. The ability to bring out the best in others and bring peace is a much better test.