The disciples had seen a man effectively casting out demons in the name of Jesus, causing harm to the kingdom of darkness and great benefit to the kingdom of God. Because this man was not part of their "group" and had not been known by them to be following Jesus, they tried to stop him. Jesus rebuked them and instructed them not to hinder him.
"He who is not against you is for you," Jesus told John. We will look at this in more detail over the next few days, but this word "against" is an interesting word and we do well to look at it a little closer. The word translated as "against" is kata and is used 328 times in Scripture. It is translated as "against" 58 times and as "according to" or "after" 168 times. Perhaps what Jesus was saying is that there will be believers who see things and do things that are a little different from the way you do it, but that does not mean they are not part of the kingdom of God. It does not mean we have the right to criticize or condemn nor to exclude them from fellowship.
This does not mean that you can believe anything you want or do anything you want. There are areas, however, that bring great controversy but should not. One example is worship style. In most of the churches I've attended, the worship style is to sit in your seat and be both quiet and still. This is pretty confining for me, but unless God moves me to join another part of the body of Christ, I'm sticking it out. I will have plenty of time to move around and shout in heaven. (Just so you know, we will all be doing that in heaven, according to Scripture)
I have much loved friends and brothers and sisters in Christ who have a freer style of worship. Clapping and a lot of motion is distracting to me, but they are accustomed to it. Neither of us is "wrong". I have a friend who dances while she worships. She doesn't call it dancing, but it looks like that to me and it is beautiful. She has a graceful, flowing motion before the Lord that is in time with the music and is beautiful to watch. It is a form of prayer for her, and I wish I could have the same freedom of motion. Again, neither my stillness in prayer nor her movement in prayer are "wrong" and Christ would not have us restrain either.
It was Jesus who said, "He who is not against you is for you." It was not the world at large to which He was referring. It was to fellow believers. The body of Christ is one body. There is not a Baptist body of Christ, a Methodist body of Christ, a Pentecostal body of Christ. We are ONE body, and as such we should function in unity, regardless of which sign is on the building where we worship.