Luke 9:49-50

John answered, “Master, we saw someone casting out demons in your name, and we tried to stop him because he is not a disciple along with us.” But Jesus said to him, “Do not stop him, for whoever is not against you is for you.”

It must have been quite the scene when the disciples tried to stop some random person casting out demons in the name of Jesus. Whoever it was - was succeeding. If demons are fleeing at the name of Christ, does it really matter who is doing the casting out? John and the others want to "corner the market" on casting out demons to "disciples of Christ". Jesus says in Matthew 7:21-23, “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven. Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?’ Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!’” The mark of a disciple is doing the will of the Father.