To a certain extent, most are. I can't really say everyone because that is rarely true. However based on my observations with the believers around me and of those I read about.. most if not all were brought up from childhood being indoctrinated into the belief. At one time, they believed in Jesus and Santa.. eventually the Santa belief gave way, mostly because adults finally stopped portraying it as true and came clean at a certain age when the child started questioning. With religion, they are admonished for questioning and the belief reinforced. So in my opinion, most of it stems from upbringing and cultural influence until it is ingrained in their minds.
Sounds like your experience is pretty limited. I grew up from childhood being indoctrinated into Christianity, which is no different than being indoctrinated into every core set of beliefs that are a part of parenting. Every parent does it. Did that prevent me from eventually making my own decision? Not at all. I actually spent a large portion of my young adulthood reading about different religions. I was ultimately satisfied that my parents got it right.
I agree that some people are admonished for questioning beliefs. I disagree that this constitutes all or a majority of Christians. There is a lot of discussion and debate about various aspects of Christianity. I'm sure that is true of many other religions, except Islam, where disagreement can cost you your life, or a body part, or both.
I've met countless educated, intellectual religious people from different faiths. I don't consider any of them brainwashed.
