In your opinion what causes christians to believe in Jesus

[John 6IX Breezy]

Many Christians assume they're saved, regardless of whether that assumption is theologically sound or not. So I don't see the threat of hell as a counter-balance to the promise of heaven.

I'm not sure that's true. I think only Pentacostals are known to have a "once saved always saved" theology. In the rest of Christianity salvation is always conditional, and sometimes painfully so. In fact, I think it can lead many to depression when not careful. You can see it in a lot of Christian hymns and prayers, this sense of being unworthy, self-deprication, and gloom.