Summary: Jesus told his disciples (and us) to have salt within us, and peace between us.
John said to him, "Teacher, we saw someone casting out demons in your name, and we tried to stop him, because he was not following us."
But Jesus said, "Do not stop him; for no one who does a deed of power in my name will be able soon afterward to speak evil of me. Whoever is not against us is for us. For truly I tell you, whoever gives you a cup of water to drink because you bear the name of Christ will by no means lose the reward.
If any of you put a stumbling block before one of these little ones who believe in me, it would be better for you if a great millstone were hung around your neck and you were thrown into the sea.
If your hand causes you to stumble, cut it off; it is better for you to enter life maimed than to have two hands and to go to hell, to the unquenchable fire. And if your foot causes you to stumble, cut it off; it is better for you to enter life lame than to have two feet and to be thrown into hell. And if your eye causes you to stumble, tear it out; it is better for you to enter the kingdom of God with one eye than to have two eyes and to be thrown into hell, where their worm never dies, and the fire is never quenched.
For everyone will be salted with fire. Salt is good; but if salt has lost its saltiness, how can you season it? Have salt in yourselves, and be at peace with one another." Mark 9:38-50 (NRSVA)
Jesus tells his believing followers to have salt within them, and peace between them. And this unusual combination follows Jesus warning the disciples about the kind of putrified attitudes and actions that characterize the inhabitants of hell. All that started when the disciples reported how they tried to close the “club membership” to one who wasn’t like them. What does it all mean?
Each week on Project Runway, Heidi Klum says, as you all know, in fashion one day you’re in and the next day you’re out. Apparently, the disciples are fans of Project Runway, wanting all their competition "out." 
We can understand the attitude of the disciples over the “outsider” doing mighty works in the name of Jesus; they were just trying to protect their turf. They couldn’t do a good job of casting out demons, so they didn’t want to be shown-up by a maverick believer who wasn’t on their membership roll.
We can also understand the warning about hell – any day in heaven with one arm is better than any day in hell with two. The warning is that hell is THAT bad. The picture Jesus gives is the local garbage dump where there is always fire, smoke and worms consuming whatever is present.
Jesus says, Cut it out! Get rid of your pride and stop making roadblocks for people who just want to serve me. Boys, boys, not EVERYBODY needs to be just like you!
A Pastor shared how he …was in a small rural church
one time that had a major dispute about where the pies should be placed in the kitchen prior to serving them for the annual turkey supper. One woman actually left the church community because several new comers to the church had convinced the rest of the women working in the kitchen that it would be more efficient to put the pies on the counter beside the sink instead of the counter next to the refrigerator.
"It’s not the right way to do it", she said. "We’ve never done it that way before, and I am not going to be part of doing it that way now. I won’t have any part of that kind of thing. Those new people are going to ruin this church. They don’t know anything. They aren’t even from around here." 
Jesus warned that kind of attitude just won’t cut it in the kingdom. Rather he warned that we should cut ourselves loose from that kind of thinking like we would cut out a cancerous tumor. An unholy attitude can kill a spirit faster than cancer can the body.
I read a story where a trap was set in the barn for a rat. In the morning it was sprung, but only the leg of a rat was there. The explanation was that the rat understood he would die if he couldn’t get free. The rodent chewed off his own leg and left it behind.
Jesus doesn’t advocate spiritual growth by hacking off your limbs; but he IS giving us a metaphor to demonstrate true spiritual growth by cutting-off and leaving-behind attitudes that can cripple ourselves, and in the process become a stumbling block to others.