Woe unto the world because of offences! for it must needs be that offences come; but woe to that man by whom the offence cometh! Wherefore if thy hand or thy foot offend thee, cut them off, and cast them from thee: it is better for thee to enter into life halt or maimed, rather than having two hands or two feet to be cast into everlasting fire. And if thine eye offend thee, pluck it out, and cast it from thee: it is better for thee to enter into life with one eye, rather than having two eyes to be cast into hell fire.Matthew 8:7-9
And John answered him, saying, Master, we saw one casting out devils in thy name, and he followeth not us: and we forbad him, because he followeth not us. But Jesus said, Forbid him not: for there is no man which shall do a miracle in my name, that can lightly speak evil of me. For he that is not against us is on our part. For whosoever shall give you a cup of water to drink in my name, because ye belong to Christ, verily I say unto you, he shall not lose his reward. And whosoever shall offend one of these little ones that believe in me, it is better for him that a millstone were hanged about his neck, and he were cast into the sea. And if thy hand offend thee, cut it off: it is better for thee to enter into life maimed, than having two hands to go into hell, into the fire that never shall be quenched: Where their worm dieth not, and the fire is not quenched. And if thy foot offend thee, cut it off: it is better for thee to enter halt into life, than having two feet to be cast into hell, into the fire that never shall be quenched: Where their worm dieth not, and the fire is not quenched. And if thine eye offend thee, pluck it out: it is better for thee to enter into the kingdom of God with one eye, than having two eyes to be cast into hell fire: Where their worm dieth not, and the fire is not quenched.Mark 9:38-48
The world is in a pitiful a wretched condition because of sin and its curse upon man and the whole creation. Part of that condition includes continual offenses between men. Jesus tells us in this passage that these offenses come because we live in a sin-cursed earth where the devil has an insatiable desire to cause misery and conflict whenever he can.
However, Jesus does not say that we should accept this condition passively. He reminds the disciples, and each of us, that we need to avoid being the one that is offensive to the children of God. We need to make sure that we are removing the temptations, distractions, and influences of Satan that would move us to say things or treat one of God’s children in a hurtful way.
Jesus uses very similar language to the Sermon on the Mount when He tells us to, by-all-means-necessary, remove the possibility of being the devil’s tool.
In the context of this discussion, John has said that they saw someone who was not part of their group casting out devils. Given that context we can apply what Jesus is saying in a couple of different ways. First of all, if the person is one of God’s children doing good things in obedience to God, then the disciples should not offend him by telling him to stop. Secondly, if the man is doing something wrong, God will handle it. God will not allow those who hurt His children to go unpunished or not chastised.
Either way – Jesus is saying leave them alone.