Mark 9:38-40

³⁸ 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.”

³⁹ But Jesus said, “Do not forbid him, for no one who works a miracle in My name can soon afterward speak evil of Me.

⁴⁰ For he who is not against us is on our side.

Also found in Luke 9:49-50.

The apostles had been arguing about the rank they would hold in the kingdom. It is obvious that they were thinking highly of themselves, and when they saw somebody without credentials doing what they themselves did, they told him to stop.

The behavior of the disciples all through this period is unsatisfactory. From the time when Peter contradicted and rebuked Jesus, down to their final desertion, there is weakness at every turn. And this is a curious example of it, that immediately after having failed themselves, they should rebuke another for doing what their Master had once declared could not possibly be an evil work. If Satan cast out Satan his house was divided against itself … [John] first discerned the rebuke to their conduct implied in the words of Jesus [Mark 9:37]. — Chadwick, page 255.

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The sincerity of the speaker [the one who was casting out demons], saves him from censure; the Lord merely corrects the error. He does not say “Receive him,” for the man’s motive did not appear; but the attitude of His disciples towards such an one should have at least been neutral … Whatever his intention, the man is for the time practically committed to a course of action which at least cannot be unfriendly. [In other words, if someone uses] Christ’s name in exorcism [it is] incompatible with hostile or unappreciative thought and speech of Him. — Wuest, page 190.

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[John’s] words are a response to the teaching just received. He and one or more of the other disciples, probably during their recent journey through northern Galilee, had prohibited a non-disciple from using the Master’s Name for the purpose of exorcising demoniacs. Ought they rather have welcomed him as a brother? The words, “He does not follow with us,” are a frank confession of jealousy for the honor of the Apostalate. In the light of the Lord’s words, the action had begun to wear a different aspect to the mind of John. — Wuest, page 189.

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The declaration “He that is not with Me is against Me” (Matthew 12:30) seems to conflict with the words “He that is not against us is on our side” (v.40). But they coalesce when it is recognized that Christ was the Divine Object and He brought things to an issue. The world was against Him; if a man was not, he was for Him, for there was no middle position. — Williams, page 738.

Many of my commentaries use this passage to condemn sectarianism — speaking against those in other denominations. But Jesus didn’t say anything about what the man believed — only what he was doing at the time. I think a modern example would be a church that taught spotty doctrine but provided food for the homeless. You wouldn’t forbid them to serve the community, but you would warn others about their false teaching.