In both the Old Testament (Numbers 11) and Gospel (Mark 9) readings, God's Word addresses the pride which corrupts His people to use one standard to condemn and potentially exclude others from the fellowship of believers, while using another, much easier standard when it comes to their own behavior. Both readings point out that it actually ought to be the other way around. God's people should be grateful for what God does in them and in others. In our text, Jesus calls for disciplined (salty) living, realizing that the way we live today has eternal consequences.
Being quick to judge others while being curiously uncritical of our own rotten attitudes is a truth that can be hard for Christians to perceive and accept. The truth about ourselves can hurt, like salt poured into an open wound and yet Jesus talks about "salt: how the hurt can help."