“Teacher,” said John, “we saw someone driving out demons in your name and we told him to stop, because he was not one of us.”
“Do not stop him,” Jesus said. “For no one who does a miracle in my name can in the next moment say anything bad about me, for whoever is not against us is for us. Truly I tell you, anyone who gives you a cup of water in my name because you belong to the Messiah will certainly not lose their reward.
“If anyone causes one of these little ones—those who believe in me—to stumble, it would be better for them if a large millstone were hung around their neck and they were thrown into the sea. If your hand causes you to stumble, cut it off. It is better for you to enter life maimed than with two hands to go into hell, where the fire never goes out. And if your foot causes you to stumble, cut it off. It is better for you to enter life crippled than to have two feet and be thrown into hell. And if your eye causes you to stumble, pluck it out. It is better for you to enter the kingdom of God with one eye than to have two eyes and be thrown into hell, where “‘the worms that eat them do not die, and the fire is not quenched.’ Everyone will be salted with fire. “Salt is good, but if it loses its saltiness, how can you make it salty again? Have salt among yourselves, and be at peace with each other.” - Mark 9:38-50
It is possible to quench the enthusiasm of someone by pointing out how much they still do not know. Supposedly "wiser" older Christians can do this when they see or hear younger Christians enthuse about their faith, how good a sermon was, how God aided them in this or that, etc. and they shake their heads and shake "That's not correct. If they really knew what God does they wouldn't be so happy right now." That is, older Christians can be the kill joys in the faith because they (we) have lost the sense of wonder and awe at the possibilities of walking close to God and interpret enthusiasm sometimes as naiveté.
In these verses, Jesus offers some stern words to his disciples who might be tempted to be the older sibling and "correct" the younger sibling rather than learning/relearning their sense of wonder. I wonder if is better to be "correct" about God or in awe of God. Since I tend to err on the side of "correcting" people, I think I need to hold myself back first rather than holding others back in enthusing. I want to wonder at God.