I suspect the issue is what the kid learned there. They were supposed to be focusing on the pro-social habits and socializing.

One hypothesis I have is that when you have a very bad epistemology and your beliefs consist of memorized atomic propositions, handed down from an authority figure, you eagerly want more people to agree with you, but not agree with you on everything except this one important atomic belief.

Religious parents and priests probably have this subconscious fear that the kid might go astray with their own theology. It's like you're a member of The People's Front of Judea and the person you'd really want to join you, joins The Judean People's Front instead.

The bit from The Life of Brian above is inspired by actual Marxist groups, which constantly splintered on all kinds of issues all the time. Marxist epistemology is nonsense, any explanation of it is just word salad. Dialectical materialism is a mysterious answer for a mysterious question. Therefore, if you're a member of Socialist Appeal, it's impossible to argue a member of a Socialist Party to switch allegiance, because there's no real epistemology, and therefore no possible inference.

Members of these organizations join based on incidental reasons, not independent inference. I suspect that members know that, at least at the corner or their minds. That's why at least some of them might not be as enthusiastic about recruiting new members, because every time you give your friend a book of Marx, you're afraid that they might acquire beliefs that are not entirely compatible with your set of Marxist beliefs.

This hypothesis might not apply though to this particular situation. Maybe religious parents want their kid to be devout, but not devout enough, so they become a celibate monk or nun instead of a patriarch or housewife of a traditional Christian family.