Charles Spurgeon 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)

But Jesus said, Forbid him not: for there is no man which shall do a miracle in my name, that can lightly speak evil of me.

These people were dissenters, we may say – a sort of outsiders.

And John puts forth the whole power of his apostolic authority to suppress them; and then Jesus Christ puts forth the full power of his divine authority to give them liberty to go on.