When I was an army chaplain, there were many who pushed for ecumenical understanding. This meant that doctrinal differences should not keep us from working and ministering together. We should not let denominational differences keep us from doing the work of the Lord.
Sometimes Mark 9:40 was used to support this notion. Jesus is talking about a man who is, in the name of Jesus, casting out demons. The disciples are put off by this guy because he was not one of the Twelve. He was not following Jesus as they were (Mark 9:38). They tried to stop him from doing what he was doing.
In response to the disciples, Jesus says, “For he who is not against us is on our side.” It is held that Jesus is saying that as long as somebody is not speaking out against Jesus, he is on the same team with all Christians. The fact that the man was doing things in the “name of Jesus” was all that was important.
If we take this to its logical conclusion, we could argue that Evangelicals should work with Mormons. They call upon the name of Jesus. They claim to be doing things in service to Him. One could argue that they are not “against” Christ. I remember when I pastored a church in Texas. The local Christian ministerial association was having an Easter cantata. I was surprised to find out it was being held in a Mormon church, and the Mormon choir was playing a significant part in the production. When I called the association office to voice my concern, I was told that the Mormons claim the name of Christ as we do, and we should serve the community together.
Of course, we know from Scriptures that doctrinal issues are of extreme importance. Most of the Epistles of the New Testament deal with doctrinal error. For example, the false teachers addressed in 1 John left fellowship with the Apostles because of doctrinal differences (1 John 2:19).
Whatever Jesus is saying in Mark 9:40, He is not saying that doctrine is not important. I think in the context there is a much better way to interpret what our Lord is saying. It seems that the reference to the name of Jesus is important. It not only occurs in v 38, but the idea is in vv 37 and 41 as well. Here the phrase seems to refer to believers who are doing the work of the Lord. Those who are “against us” would then be a reference to unbelievers. We see this idea in Matt 12:30. The Pharisees have rejected Christ and claim that He is possessed by Satan. Jesus refers to them as those who are “against” Him.
The whole context of Mark 9 is about discipleship. From 9:33 until 9:50 Jesus is teaching the disciples in a house (v. 33). The main point of the teaching is that disciples of Christ should not lord it over each other. They should be at peace with each other. They should serve the least among them.
The man casting out demons was looked down upon by the disciples. He was not in the inner group. But Jesus is saying that all believers who are serving the Lord are pleasing to Him. We may not know this man’s name, and he may not have been one of the Twelve, but the Lord approves of what he is doing. In the very next verse (Mark 9:41) the Lord says that people who behave as this man will not lose their reward. Even if a disciple gives a cup of water in his service to the Lord and others, he will be rewarded.
Mark 9:40 is not a call for ecumenicalism. It is a highly encouraging lesson for all of us. No matter who we are, no matter if what we do seems insignificant to the world, and even if it is insignificant in the mind of other believers, we are doing it “for” the Lord. It is pleasing to Him. All disciples are on the same team.