In your opinion what causes christians to believe in Jesus

[Alan V]

Ah, the majority. They are always right.

Don’t believer say billions of xians can’t be wrong?

That is part of the reason why believers feel so comfortable believing -- that human compulsion to conform.  If ordinary people were called upon to believe something more rarified but more accurate, like Paleophyte said, they would disappear.  

If you use conditioning as a method of socializing people, don't be surprised if they are uncomfortable with change.