If literally the only evidence you had was that the overwhelming majority of people professed to believe in religion, then you should update in favor of religion being true.
Your belief that people are irrational relies on additional evidence of the type that I referenced. It is not contained in the fact of overwhelming belief.
Like how Knox's roommate's death by murder is evidence that Knox committed the murder. And that evidence is overwhelmed by other evidence that suggests Knox is not the murderer.
Whether people believing in a hypothesis is evidence for the hypothesis depends on the hypothesis. If the hypothesis does not contain a claim that there is some mechanism by which people would come to believe in the hypothesis, then it is not evidence. For instance, if people believe in a tea kettle orbiting the sun, their belief is not evidence for it being true, because there is no mechanism by which a tea kettle orbiting the sun might cause people to believe that there is a tea kettle orbiting the sun. In fact, there are some hypotheses for which belief...
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.