People believe in God because of 3 fears that they allow to rule their lives. As we primates developed an ability to think abstractly and not only became self aware but became future aware and knew one day we would die, the fear of death caused us to invent a "soul" to mitigate the horrible thought of no longer existing at some point in time. The soul needed a creator and redeemer, thus a god was invented also.
As we became smarter, more analytical, and had significant knowledge to pass down from one generation to another we became very powerful indeed. Rather than simply fighting over territory with the tribe next door, we could plan out their demise and carry it out. But with power, also comes an awesome responsibility of which we feared. Thus, when we did something horrible, like butcher the tribe next door, or if something horrible happened to us like we died of disease because we didn't know about proper sanitation, it was "god's will." Rather than accept responsbility we now have someone we can blame it on. A couple I know recently accidently got pregnant while the girl was on some major meds for an unrelated problem. They're religious so won't consider an abortion even though the meds may have a terrible impact on the fetus. Their response? "God put us in this position" It apparently had little to do with them.
The last fear is insidious. It is a mental virus. Early in the first milennia AD, there was a big political movement to try to convert as many people as possible to christianity. So Hell was invented as a scare tactic. If you didn't have faith, if you didn't convert, if you didn't pay money to the churches, there would be Hell to pay! The movement hit critical mass and to this day, some of the first concepts children are taught is this horrible place called hell where the "loving god" sends you to be burned for all eternity if you don't blindly follow every doctrine. In short, it is a brainwashing scheme perpetrated on people at the youngest age possible, which is why religious people stay religious.
So there you have it. The causes of religious fanaticism and adherence:
(1) The fear of death
(2) The fear of responsibility
(3) The fear of Hell
