Immediately following the debate as to who would be the greatest in the kingdom of God, Luke tells us in Luke 9:49 that John replied, "Master, we saw someone casting out demons in your name and we forbade him because he does not follow us."

What's going on here? Some say that John is only offering this comment as a diversion to deflect Jesus' words of rebuke that He had just given to His disciples. Others say that no, it's an earnest concern, and that John had seen somebody actually casting out demons using the name of Jesus, but he wasn't one of the twelve or one of the seventy-two in the broader company of Jesus' disciples. And John was miffed at this and wanted Jesus to rebuke the fellow and stop him from carrying out this ministry that he was doing in the name of Jesus.

What is it Jesus sees here in John? It is a spirit that is contrary to authentic discipleship. He sees a narrow exclusiveness, a parochial attitude that basically says, "If he's not a part of our group in its purest form, he has nothing whatsoever to do with us." Does that sound familiar? Do we not commit this same offense again and again? Today, we might say, "He may claim to be a Christian, but he's not really Reformed so we can't trust him. Or, he's not an Episcopalian or a Lutheran like we are, so we can't trust him."

I don't know of anybody who's a greater fan of Martin Luther than I am. But one of the low points of the Reformation took place in 1529 when an attempt was made to unify the Reformers of Switzerland and the followers of Luther in Germany. And so, at Marburg, an important historic colloquy was held. Representatives of both sides attended, including Luther from Germany and Ulrich Zwingli from Switzerland, and they tried to hammer out a position of unity so that they could stand together for the Reformation.

After much discussion, they couldn't agree on the manner in which Christ is present in the Lord's Supper. Both sides believed that He is present, but the mode of His presence was a matter of dispute. Luther insisted on the physical, corporeal presence of Christ in the sacrament. And like Nikita Khrushchev years ago with the United Nations when he slammed his shoe on the table, Luther pounded the table saying, "Hoc est corpus meum!"—a Latin phrase that means "This is my body!" Luther believed the only way we can take these words of Jesus are in the fullest corporeal sense. Zwingli and the others said, in essence, "Wait a minute, Jesus also said, 'I am the vine,' and 'I am the door.' Can't the word is be used in a way that represents something in the Lord's Supper without this insistence on literalism?"

The two Reformers could not come to an agreement. But beyond disagreeing, the saddest thing was when Luther turned to Zwingli and said, "You are an andern Geist"—German for "a different spirit." Luther questioned Zwingli's Christianity altogether. Thus was introduced the divide between the Lutheran and Reformed wings of the Reformation that exists to this day.

At this point, I have to say, "Shame on you Martin Luther; you were just like John." Luther insisted that those who don't agree with us at every point are really not of Christ. We should learn, not only from that tragedy at Marburg, but also from this encounter in the Scriptures.

http://www.ligonier.org/blog/warning-against-division-church/