Adam Clarke on Mark 9:39

Forbid him not - If you meet him again, let him go on quietly in the work in which God owns him. If he were not of God, the demons would not be subject to him, and his work could not prosper. A spirit of bigotry has little countenance from these passages. There are some who are so outrageously wedded to their own creed, and religious system, that they would rather let sinners perish than suffer those who differ from them to become the instruments of their salvation. Even the good that is done they either deny or suspect, because the person does not follow them. This also is vanity and an evil disease.