Albert Barnes Commentary Mark 9:38

"John said unto him, Teacher, we saw one casting out demons in thy name; and we forbade him, because he followed not us." — Mark 9:38 (ASV)

We saw one, etc. There is no improbability in supposing that this might have been one of the disciples of John, or one of the seventy whom Jesus had sent out, and who, though he did not personally accompany Jesus, yet had the power of working miracles. There is no evidence that he was merely an exorcist, or that he used the name of Jesus merely as a pretense.