As someone raised in an authoritarian Calvinist belief system (albeit not in the US) it’s simpler than you think in my opinion. People in general believe whatever fits the random things they absorb into their psyches growing up as kids because being kids they have no yardstick to differentiate between fantasy and reality other than what the authority figures in their life tell them. This is true whether you’re raising someone on a diet of intellectual curiosity, a diet of pernicious cult-adjacent nonsense, or anything in between. There’s a reason so many people who’ve left similar authoritarian beliefs love “The Truman Show” in my experience but it’s true for everyone.

Part of being human is that our perceptions unconsciously bend around our beliefs and that the stronger the belief the stronger the amount of bending. We all have this built-in bias towards existing beliefs to a greater level than we instinctively assume, although it’s usually a lot more obvious among authoritarian religious groups because their beliefs are so markedly different from consensus reality.