"'Teacher,' said John, 'we saw someone driving out demons in your name and we told him to stop, because he was not one of us.'" NIV translation
John tells Jesus of an incident in which the disciples stopped a man from casting out demons in the power of Jesus. This seems rather strange when we remember that, in the place where they just left, the disciples had not been able to drive out a demon. We see that the reason that they told him to stop was because he was not one of their group.
The same type of thing happens even today within the church. Instead of working together, local bodies of believers will actually compete with each other. They will even go so far as to attempt to eliminate another ministry or to claim the work that another has started.
"'Do not stop him,' Jesus said, 'For no one who does a miracle in my name can in the next moment say anything bad about me, for whoever is not against us is for us.'" NIV translation
Jesus' reaction was to explain that it is not about their group but about His power. John and the disciples had lost sight of the goal of proclaiming the way of salvation. As we saw with their arguing over who was the greatest, they were more concerned with their pride than with the work of the kingdom. Sadly, this same pride is even more rampant today as we see the huge number of divisions and denominations in the body of Christ. This could all be eliminated if we would only remember the answer that Jesus gave to John.
"'Truly I tell you, anyone who gives you a cup of water in my name because you belong to the Messiah will certainly not lose their reward.'" NIV translation
Jesus' answer is simple in the fact that He reminds John that He is in charge and is the one that will hand out the rewards. As Christians, we will all answer for our words and actions. This will not be a judgement of punishment but a judgment of rewards. Believers should not think that they will be rewarded for trying to steal the rewards of another believer. That is, in effect, what John was describing and it was based on pride.