Unfortunately it is common for people of different denominations and Christian sects to argue with each other. History is sadly filled with examples of the body of Christ dividing and being made ineffective because of disagreements between believers. Families and even nations have broken up because Christians couldn't overdome their differences and focus on their similarities.
This is NOT what Jesus wanted.
Yes, it is important that we stand firm on the core beliefs of Christianity and the teachings of Christ as presented in the Bible. Many of Paul's letters dealt with incorrect beliefs and actions of the church. However, he didn't automatically turn his back on those churches or say that they should be rejected.
Because of our sinful human nature, Christianity is often a clique of sorts, where we gather together with like-minded individuals, pushing away any who don't subscribe to our particular "flavor" of belief and worship. If someone isn't a part of that group, we don't want them around. Does that sound hard to believe? It even happened to the Apostles.
An argument started among the disciples as to which of them would be the greatest. Jesus, knowing their thoughts, took a little child and had him stand beside him. Then he said to them, “Whoever welcomes this little child in my name welcomes me; and whoever welcomes me welcomes the one who sent me. For it is the one who is least among you all who is the greatest.”
“Master,” said John, “we saw someone driving out demons in your name and we tried to stop him, because he is not one of us.”
“Do not stop him,” Jesus said, “for whoever is not against you is for you.” Luke 9:46-50
Jesus' own followers, his core 12, were arguing over which of them was the best. Remember when the mother of James and John tried to get them at the Lord's side over the other Apostles? Some of the disciples had a problem with thinking that they were the greatest, and the others were less. Some felt that the others who walked with Christ were somehow less important, less effective, or had differing beliefs. Rather than remembering that ALL of them were personally chosen by the Messiah, they bickered over who was the best or favorite.
Christianity would be far more effective if we would get past our differences and work together to further God's Kingdom. Can you imagine what would happen to the world if Catholics, Lutherans, Baptists, Methodists, Pentecostals, Presbyterians, nondenominationals, and other Christians banded together in unity? How would all nations in the world be moved if we started fellowshiping with those who might have different traditions and views on Scripture, and concentrated in sharing the Good News of Jesus as the Messiah? Unfortunately this doesn't happen nearly often enough. These pet beliefs are so precious to some that anyone who doesn't share them must be an outsider to be shunned.
That's what happened when John and the others saw someone chasing demons in the name of Christ. Because this man was not one of the Twelve, they tried to stop him. He was doing the right thing in the name of the right person, but because he wasn't part of the "elite", he was to be condemned. And how did Jesus respond? He chided his disciples, and basically told them that it didn't matter that he wasn't part of their little group. Since he was trying to do the same thing that they were, and in the name of Christ, they should accept him.
Keep this story in mind the next time you come across someone who is a Christian, but maybe sees things a bit differently than you. Perhaps you are a political conservative and they are a liberal. Maybe you believe in infant baptism but they don't. Or you think that having alcohol is fine but they say it will lead to sin. Some may differ on how the Bible says we should treat homosexuals. In these situations and many, many more, remember that despite such differences, we all follow Christ and want others to find a place with Him. We need to work together, not push each other apart. In Jesus' own words, whoever is not against us is for us!