In your opinion what causes christians to believe in Jesus

[John 6IX Breezy]

That works great on kids, less great on teens.

Yes, that's because it doesn't matter how much conditioning you think you've done as a parent towards your three-year-old, eventually they'll reach adolescence and begin treading their own path. I'm sure quite a few people here either left or adopted religious views around 12-14.

p.s. I think it's good you don't use words like indoctrination. But keep in mind that conditioning comes from the defunct school of behaviorism. It is still a real aspect of learning, but all the implications and inferences that it used to have are wrong.