In your opinion what causes christians to believe in Jesus

[Angrboda]

In psychological terms, they believe because they converted for emotional reasons -- either a cathartic experience as an adult, or as a child who needs to minimize anxiety by adopting the commands and instruction of their parents -- they began to believe, and once believers, the inertia of belief made it more probable that their belief would intensify over time rather than weaken.

From a more fundamental aspect, belief likely has two things going for it. First, we are evolved to posit the existence of minds that we cannot see in other persons, making it easy to postulate a similar thing for non-human subjects, like waterfalls, streams, the wind, and ultimately to postulate a grand mind above everything else. The second follows on the first in that because the mind is not seen nor tangible, it's natural to presume it continues on past death. This has been shown in children wherein they possess confidence about the nonexistence of an agent's physical embodiment when it is killed, but are less sure about the mental or mind aspect of an agent after death. These two likely work in concert with an overall need to minimize uncertainty about the environment to make the idea of a great mind ensuring order and justice in the world an idea which is both appealing in how it offers us comfort and less anxiety as well as fitting with other intuitions (such as fairness, which when addressed by a just world belief, assured by a god, allows a contentment about existence which a belief in an ultimately unjust world would not).

As to the specific character Jesus, hero worship, wherein we conflate the ordinariness of a person with a mythical greatness seems to be a common narrative theme. Thus we likely gravitate toward belief in hero figures. And if said hero figure promises the above benefits of maintaining justice and order in the world, they're likely to be even more popular.