Charles Spurgeon Commentary Luke 9:50

"But Jesus said unto him, Forbid [him] not: for he that is not against you is for you." — Luke 9:50 (ASV)

Jesus also said that no one could perform a miracle in his name and then lightly go and speak evil of him; so that it was for the good of the cause to let the irregular practitioner continue with this business.

Besides, if anyone can cast a devil out, by all means let them do it; for there is no great abundance of the power to cast out devils; and, remember, these gentlemen who found fault could not cast the devil out themselves. They had failed at this very task; and yet, when someone else did it in the power of God, they began to complain and forbid them. That is surely being like the dog in the manger. God save us from falling into that spirit!