Luke 9 contains the story, sometimes referred to as the unknown exorcist, of a man unknown to the disciples but who is casting out demons in Jesus’ name.

John answered, “Master, we saw someone casting out demons in your name, and we tried to stop him, because he does not follow with us.”

Luke 9:49

The disciples had just been sent out and they see someone else performing their own ministry, but as an outsider from Jesus and the twelve disciples.

We aren’t given direct details about the backstory, but we can make an education guess as to what happened. Earlier in the chapter, the twelve are sent out to preach and to heal:

And [Jesus] called the twelve together and gave them power and authority over all demons and to cure diseases, and He sent them out to proclaim the kingdom of God and to heal. ...

And they departed and went through the villages, preaching the gospel and healing everywhere.

Luke 9:1,6

It’s reasonable to assume that people observed what the disciples were doing and how they were doing it, and in this case the man seems to be casting out the demons in the way he saw the disciples do it. A if it ain’t broke don’t fix it type of theology, and it would appear to be successful.

Interestingly, Jesus states:

But Jesus said to him, “Do not stop him, for the one who is not against you is for you.”

Luke 9:50

(Side note: I have to believe this is a case for Christian unity, in particular that cuts across secondary and tertiary beliefs and traditions, that otherwise would divide different churches and denominations.)

There’s a similar story in Acts 19 where the seven sons of Sceva attempt to cast out demons:

Then some of the itinerant Jewish exorcists undertook to invoke the name of the Lord Jesus over those who had evil spirits, saying, "I adjure you by the Jesus whom Paul proclaims."

Acts 19:13

A key distinction here is that the seven sons attempt to cast out demons by the Jesus whom Paul proclaims. The seven sons are one step removed from Jesus and are attempting to piggyback on Paul’s faith, but it doesn’t work well for them:

But the evil spirit answered them, “Jesus I know, and Paul I recognize, but who are you?”

Acts 19:15

If the demon knows Jesus and of Paul, but doesn’t know the sons, do the sons know Jesus? When we know Jesus, we operate under his power and authority:

"Truly, truly, I say to you, whoever believes in me will also do the works that I do; and greater works than these will he do, because I am going to the Father. Whatever you ask in my name, this I will do, that the Father may be glorified in the Son. If you ask me anything in my name, I will do it."

Luke 14:12-14