It is part of Christian doctrine that only Christians can be saved. No one comes to the Father except by Christ (John 14:6). This is a view that can encourage tribalism.
In the Gospel Reading, the apostles see someone who is not among the group of Christ’s disciples but who is casting out demons in the name of Christ anyway. When the apostles perceive the man doing a miracle in Christ’s name, they tell him to stop it. As the apostles see it, unless a person is a member of their group, their tribe, he is an outsider; and, as an outsider, he is not among the saved. In the view of the apostles, that is why there is something wrong about that outsider’s using God’s power to do miracles.
In response, Christ defends the outsider and rebukes the tribalism of his apostles. “Whoever is not against us is for us,” Christ tells the disciples. The outsider has not joined the disciples, and he doesn’t know the doctrines or customs that the disciples are learning from Christ. But, in Christ’s view, he is still one of Christ’s own.
So it is true that no one comes to the Father except by Christ.
But this requirement makes Christ the one necessary and sufficient condition for salvation. It doesn’t say anything about any of the other ways by which members of a group usually identify themselves. As Christ sees that outsider, the man meets the one requisite condition for salvation: he is for Christ. And so, in Christ’s view, that outsider counts as one of those who come to the Father by Christ, even if he is not an official member of Christ’s disciples.
But then, you might say, who doesn’t count as one of Christ’s own? Who actually is against Christ?
In effect, Christ answers these questions by giving an example of a group destined for perdition. It is the group of those who draw children away from the Lord or who make the vulnerable and helpless worse than they otherwise would be. Those in that group would be better off being dropped into the sea with millstones around their necks, Christ says.
So here is what we need to remember. Even Judas was a member of the official tribe of Christ’s disciples; but in the end Judas was not one of Christ’s own. Real membership in the group of those who are Christ’s own depends on being for Christ, and for those he loves.