Thank you for asking about Mark 9: 38-40: "John said to him, 'Teacher, we saw a man casting out demons in your name, and we forbade him, because he was not following us.' But Jesus said, 'Do not forbid him; for no one who does a mighty work in my name will be able soon after to speak evil of me. For he that is not against us is for us.'"
I'm reminded of a similar passage in the Book of Numbers (11:26) in which Joshua complains to Moses that two men not authorized by Moses are down in the camp, essentially prophesying without a license. Moses replies (with a sigh, I think) "Would that all the Lord's people were prophets!"
I think both passages tell us that those who are most intimately linked with, and expressive of, the Presence and Power of God are the least likely to be possessive or authoritarian about it. The entire point of Jesus' ministry was to awaken us to our own spiritual power and potential. Once we have that awareness, we will make better, more loving choices and thereby bring into expression the great spiritual potential that Jesus describes as 'the kingdom of heaven.' What Jesus taught are universal spiritual truths; they are not the exclusive property of any particular church or religion. And Jesus is here clearly saying that what matters are the results, not who gets credit. It's all God!