A lot of it is brainwashing. From their early years, they're usually raised by fanatical parents who would disown them if they questioned their faith. Then there's the schools. When I was a kid, I went to a Christian school for six months (my parents aren't religious it was just close by) if you didn't say a prayer before eating lunch, you were in trouble,If you didn't sing along with the hymns, you were in trouble. Even miming would piss them off. If they could sense disinterest in what they were preaching, you were in trouble. They hated independent thought, and a student with his own views was more or less an enemy of the teachers and was vilified for their beliefs. It was pathetic. It wasn't "do you believe?" instead it was "you WILL believe, whether you want to or not." I can't even imagine what the religious schools must be like in the states. It makes no difference to me if someone is religious but I think it's ludicrous that some of them go to such extremes to force their beliefs on others.
The sooner mankind is extinct