"Now John answered Him, saying, 'Teacher, we saw someone who does not follow us casting out demons in Your name, and we forbade him because he does not follow us.' But Jesus said, 'Do not forbid him, for no one who works a miracle in My name can soon afterward speak evil of Me. For he who is not against us is on our side. For whoever gives you a cup of water to drink in My name, because you belong to Christ, assuredly, I say to you, he will be no means lose his reward'" (Mark 9:38-41).
The apostle John complained to Jesus about a man who was not a part of their immediate company. In other words, he was unknown to them and did not travel with them. The man was casting out demons in the name of Christ, but the apostles forbid him from continuing to do such. Evidently, the man had miraculous power and his use of it stirred up jealousy in John and the other apostles. They likely thought that no one else besides the chosen twelve should be honored with this power. A similar attitude was mistakenly expressed by Joshua in Numbers 11:27-29.
Jesus responded - "Do not forbid him, for no one who works a miracle in My name can soon afterward speak evil of Me" (Mark 9:39). If the man had been an enemy of Jesus, using his power in opposition to the truth, it would have been right to forbid him. But, according to John's own statement, he was casting out demons in the name of Jesus, and thus he was a friend. Surely this man could not have cast out demons by Jesus' authority (i.e., His name) unless the Lord had given him the power to do so, and if Jesus had given him the power then it was his privilege to use it to God's glory!
"For he who is not against us is on our side" (Mark 9:40). The converse of this statement is found in Matthew 12:30 - "He who is not with Me is against Me, and he who does not gather with Me scatters abroad." These two verses together clearly show the impossibility of neutrality. If a man is in no sense against Christ, then he is for Him; if a man is not for Christ in all aspects, then he is against Him.
In Mark 9:41, Jesus returns to the discussion of greatness and asserts that the smallest act of righteousness, if done for the sake of the Lord, will be honored in the kingdom (cf. Matt. 10:40-42).
This context has long been abused by many in order to justify cooperation and fellowship between God's people and denominations. It is generally claimed that since the denominations believe in Jesus Christ and are actively engaged in good works of benevolence, then they are not against us--that is, members of Jesus' church. Since it is reasoned that these denominationalists are not against us, but on our side, we should therefore not "forbid" them but embrace them. Some will even go as far as to say that to speak out against denominations is parallel to committing the sin John and the other apostles did here.
This interpretation is severely flawed, however. This man, whom the apostles had rebuked, was doing that which God had authorized him to do. Although we don't know the details of how he got this miraculous power, it is certain that it was from God and was being used in accordance with His will. Thus, he should not have been rebuked. However, no member of any modern day denomination is fully doing that which God would have him to do for no denomination even has the right to exist (cf. I Cor. 1:10; John 17:20,21)! God's desire is for spiritual unity based upon His truth, but denominationalism is religious division caused by human traditions and the opinions of men! It is true that denominations often engage in many good works (from a moral perspective), but this doesn't justify overlooking their doctrinal errors which they teach and practice, and from which they need to repent (cf. II John 9-11).