Jamieson, Fausset and Brown on Mark 9:40

40. For he that is not against us is on our part--Two principles of immense importance are here laid down: "First, No one will readily speak evil of Me who has the faith to do a miracle in My name; and second, If such a person cannot be supposed to be against us, ye are to consider him for us." Let it be carefully observed that our Lord does not say this man should not have "followed them," nor yet that it was indifferent whether he did or not; but simply teaches how such a person was to be regarded, although he did not--namely, as a reverer of His name and a promoter of His cause.