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 see this outsider doing a miracle in Christ’s name, they tell him to stop it. Unless he is a member of their group, their tribe, they think, that outsider is not among the saved. And if he isn’t, then, in the view of the apostles, there is something wrong about his 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 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 other tribal ways by which members of a group usually identify themselves. The outsider meets the one requisite condition for salvation, and so he 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. Real membership in the group of those who are Christ’s own comes from being with him and for him.