And Jesus said unto him, Forbid [him] not: for he that is not against us is for us.
Note 2 at Lu 9:50: This man whom the disciples rebuked was doing the works of Jesus (Joh 14:12), not the works of the devil. Jesus was saying that those who are doing God's work are on our side even if they aren't doing it exactly the way we do (see note 10 at Mr 9:38). We shouldn't rebuke them.
Actions are the sure test of whose side we are on (1Jo 2:29; 3:7, and 10). A contrasting scripture in Mt 12:30 says, "He that is not with me is against me." If we call Jesus Lord but don't do the things He tells us to do (Lu 6:46), our profession is worthless. "Faith without works is dead" (Jas 2:20). "Wherefore by their fruits ye shall know them" (Mt 7:20).