What does Mark 9:38 mean?

The verse feels like a non-sequitur: a statement with broken logic. However, it's part of the longer conversation about where individuals fit in to Jesus' kingdom. The disciples assume they are first and will have the most authority. As a result, they argue to see where they rank with respect to each other. Jesus explains that leaders are first and foremost servants. They should be more concerned about welcoming the powerless, like children, than displaying their own power (Mark 9:34–37). So they ask about someone who publicly appears to be in their group but isn't. Jesus says they're still being too exclusionary. If this man has faith enough to exorcise demons in Jesus' name, he's not a threat to the kingdom, even if he may be a threat to the disciples' egos (Mark 9:17–18).

John is also telling in his wording, "he was not following us." There is no "us" to follow: there is only Christ. Paul talks about this when he chastises the Corinthians for claiming to follow a human leader—Apollos or Peter or himself (1 Corinthians 1:12). Paul points out that such thinking leads to ungodly tribalism when, really, they are all servants of the Lord (1 Corinthians 3:3–5). The fact that this man was, apparently, successful where the disciples themselves had recently failed would have also been a blow to their pride.

How often do we discourage someone who has success where we failed? Especially if they're not in our established system? The disciples' attitude is very close to that of the scribes and Pharisees who reject Jesus not because He works against God but because He isn't in their exclusive club (Mark 3:22).