This is all OPINION.
As a child? It isn't intentionally malicious or anything like that, but the parents and the adults the child trusts. The church gets them young and before the schools teach them critical thought. They have no life experiece yet, how could they possibly tell the difference? Children will also believe in fairy tale creatures like the easer bunny, tooth fairy, santa claus if you tell them they're real. By the time they reach their teens they've been in the church and the church culture their entire life. I think this can apply to any religion anywhere.
Adults I can only give you my own experience so it may or probably doesn't apply everywhere. I grew up in a small Baptist church in a town in northern Canada. For single people often it was people who fell out of the faith returning, or they hit a rough spot in their lives and filled it with lies including jesus as motivation to keep living. Whether it was drugs, depression, divorce or whatever caused it. I never saw any single adults joining who weren't at least mildly fucked up in some way. The thing is, when you join you gain an instant support group. I can see why that would be appealing. Couples were usually newly moved to the town and/or just had kids and wanted to add some god to their lives but also to start socially networking, because why not start them before they have any way to opt out or know better? If they were pre-teens or teens usually they were brought in by their friends, the ones who stuck around were almost always socially awkward or weird kids who really like the postive attention on them possibly more so than they actually believed in the relgion...at first. Eventually they started to really believe too. Added perk of the church members liking to marry eachother, a paradise for the weird.
Who controls it? Nobody anymore! Not directly. Sure you can use it to gain favor from the population, but you aren't in charge of it. How did it start? Conquer a region, make current religion illegal or worse. Then make up some rules that favor you personally and outlaw things you don't like. Then fuck some kids. Then die. Wait 2000 years. Viola, modern day religion.
You can also start one by starting a cult, then being dead and wait a couple centuries. Viola, Mormons!
I bet Scientology will one day have a leader that ACTUALLY BELIEVES in Xenu and the church teachings.