Why were the disciples arguing about which of them was the greatest? Peter, James, and John had recently witnessed the Transfiguration of Jesus on the mountaintop; likewise they witnessed Jesus cast out a demon they themselves had failed to cast out, and then they were puzzled at Jesus’ second prediction of His Passion. Perhaps they were wondering who might lead them if Jesus truly were going to be taken away or even killed? Perhaps the three who were atop the mountain while the other nine failed to cast out demons were thinking they were special? Whatever the case, Jesus swiftly ends the argument by reminding them of the importance of humility, of being childlike. As St. James would later write—perhaps remembering this incident from his own experience—“Humble yourselves before the Lord and he will exalt you” (Jas 4:10).
And how does He humble the Twelve? John says they saw others casting out demons in Jesus’ name and succeeding, thinking He would put a stop to it, or denounce it. He does not. They who have followed Jesus failed to do what those who were not following Him did perhaps because they had not the humility to be victorious.