Theophylact of Ohrid
Catena Aurea · Lukasevangelium 9
Then there arose a reasoning among them, which of them should be greatest. And Jesus, perceiving the thought of their heart, took a child, and set him by him, And said unto them, Whosoever shall receive this child in my name receiveth me: and whosoever shall receive me receiveth him that sent me: for he that is least among you all, the same shall be great. And John answered and said, Master, we saw one casting out devils in thy name; and we forbad him, because he followeth not with us. And Jesus said unto him, Forbid him not: for he that is not against us is for us.
Because then our Lord had said, He who is least among you all, the same shall be great, John feared, lest perhaps they had done wrong in hindering a certain man by their own power. For a prohibition does not shew the probitor to be inferior, but to be one who thinks himself somewhat superior. Hence it is added, And John answered and said, Master, we saw one casting out devils in thy name, and we forbad him. Not indeed from envy, but to distinguish the working of miracles, for he had not received the power of working miracles with them, nor had the Lord sent him as He did them; nor did he follow Jesus in all things. Hence he adds, because he followeth not with us.