What does Luke 9:50 mean?

The disciples saw a man casting out demons in Jesus' name. Since they don't recognize him, they tell him to stop (Luke 9:49). The disciples think they are preparing for the kingdom of God within the kingdom of Israel. Jesus has a much bigger task in mind: the kingdom of God throughout the world in the body of the church.

Jesus' kingdom is inclusionary. The disciples will be leaders in the church, but they cannot complete the task alone (Luke 10:2). Priscilla and Aquilla knew this when they found Apollos forcefully teaching the parts of Jesus' story he knew. Instead of declaring him a false teacher, they took him aside and trained him (Acts 18:24–28).

Jesus has shown the disciples how to identify someone who is with Him: they honor the powerless and willingly serve all (Luke 9:48; Mark 9:35). This man, casting out demons when the disciples lost their ability (Luke 9:40), expresses the concern for downtrodden people which Jesus looks for.

By placing this story right after the disciples' argument over who is greatest (Luke 9:46–48), Luke also sends a message: the disciples still don't understand their role in God's kingdom. They think they are protecting Jesus' "brand:" as if He were a corporate trademark or a patent. In their minds—at least, for now—there's no room for anyone who isn't in the inner circle. Paul will later write to the Corinthian church:

For it has been reported to me by Chloe's people that there is quarreling among you, my brothers. What I mean is that each one of you says, "I follow Paul," or "I follow Apollos," or "I follow Cephas," or "I follow Christ." Is Christ divided? - 1 Corinthians 1:11–13a

The disciples will learn that the group is designed to encourage Christ-followers and help them grow, not to protect their precious positions.

We don't know anything about this man. Apparently, he isn't a part of the larger group of disciples who periodically travel with Jesus (Luke 10:1–12). He has enough faith in Jesus to expel demons, but we don't know how much he understands who Jesus is.

This marks the end of the section some Bible scholars call "Jesus' Ministry in Galilee." Not all the events occur in Galilee, but this section includes the core of Jesus' ministry to the crowds, especially the miracles. In Luke 9:51, Jesus "[sets] his face to go to Jerusalem." Luke's account shifts into an intense period of Jesus training the disciples for what they can expect in the future. They don't believe that Jesus will be betrayed and turned over to religious leaders who ensure He meets death on a cross. But they need some foundation when they do finally believe. In addition, they need strong training on what the kingdom of God really is so they will be ready to build the church.

What scholars sometimes refer to as the "Travelogue to Jerusalem" includes stories and teachings to that end (Luke 9:51—19:27). After the travelogue, Luke recounts Jesus' arrival in Jerusalem, the crucifixion, and the resurrection. Luke also wrote the book of Acts to show Theophilus (Acts 1:1–3) how the disciples used Jesus' teaching. They expanded God's kingdom through the church from Jerusalem all the way to Rome.