Expositor's Bible Commentary Commentary Mark 9:39

"But Jesus said, Forbid him not: for there is no man who shall do a mighty work in my name, and be able quickly to speak evil of me." — Mark 9:39 (ASV)

Jesus did not have as restrictive a view of who could legitimately participate in his mission as his disciples did. The casting out of demons was done by God’s power, and his power was not limited to the Twelve. So Jesus told his disciples not to stop the strange exorcist because such a man was not likely to speak badly of Jesus if he performed a miracle in his name (cf. Numbers 11:26–29).

Casting out demons definitely demonstrated that the man was not against Jesus, and “whoever is not against us is for us.” How does this relate to Mt 12:30, “he who is not with me is against me”? Jesus did not want to force people quickly into a decision about himself. He desired to give them plenty of time to decide, during which the principle in Mk 9:40 applies. But when the critical moment for decision arrives, then the principle laid down in Mt 12:30 takes over.