In your opinion what causes christians to believe in Jesus

[Alan V]

I've got one. Why is anyone still giving Johnny attention?

Because people feel they are defeating and humiliating him, and they enjoy doing this and then gloating about it.

Or perhaps because this is an atheist forum, and each arrogant religious person who intrudes here provides the opportunity for resident atheists to again show how stubbornly irrational some religious people can be.  Aside from the taunting on both sides, such discussions help promote atheistic perspectives.