I have the "child-like" way of thinking too sometimes like assuming that people are of the belief that God is real or that they love God because they say his name in a nice or respectable way or call out to him in times of stress then, find out later that that isn't the case at all and then, it hurts and makes me feel hopeless. I have felt similar feelings over other behaviors, mannerisms and words used by others only to find later that I was way off. I had arrived at the conclusion since then that I was just ignorant and had never had things explained to me properly as I grew up which is true in part because I was just treated by my family like I was supposed to know things automatically without being told which brought me to the conclusion that I have no common sense. This too is true in a way. Stuff that everyone else just 'gets' immediately has to be explained to me in great detail sometimes in order for me to understand. Sometimes I tell people to explain what something means as though they were talking to a dummy or a child so, that I will 'get' it.
I never considered that this may be part of a mental problem though.
Interesting. Learn something every day I guess.