It could have been that Mark recorded the incident in today's Gospel as a warning against a tendency towards exclusiveness that he may have noticed in the early Church that he knew. We should not be too surprised if such small cliques did exist because the early Christians were small groups surrounded by indifferent or even hostile neighbours. They needed to stick together for mutual support, and in their beginners' fervour may have been blind to even the possibility that goodness could exist outside their groups. Such an attitude would not have been according to Jesus' mind.
In the Book of Numbers (chapter 11) there is a refreshing lesson in tolerance. Joshua, the fervent young aide to Moses, was upset by Eldad and Medad prophesying to the people even though they weren't members of the seventy elders surrounding Moses. He asked Moses to stop them. Moses gave him a wonderful answer," Are you jealous for my sake? Would that all the people of God were prophets.
We believe and act on that belief that the Church is God's way of salvation for all. We must, however, recognise that in actual life, God's love has to go beyond the boundaries of the Church and find people where they are.
Heavenly Father grant me the grace to show Your loving Face to those who are seeking You but have not yet found You.