But I think, in your comment, what you're doing is conflating intelligence with rationality. Raw intelligence doesn't help you to notice when you have an ingrained-but-senseless belief (thus predicting precisely your "candidates who are esteemed scholars, scientists and technologists, who are nevertheless fervently religious".) One of the primary properties of modern religion is its non-falsifiability -- it makes no predictions about your everyday experience you could keep track of to form a sense of "how it's doing." Because of this, a religion can sit comfortably in your mind alongside everything else you "know", and no piece of evidence you run into will ever rub up against it and wear it away.
Noticing that you have beliefs that aren't causally attached to the rest of your experience is a skill which you have to learn explicitly. This set of skills (instrumental rationality) together are a multiplier for the productive output of your intelligence, but they don't make you any more or less intelligent. Really intelligent people might come up with these skills on their own, as meta-patterns that seem to hold between pattern in different disciplines, but most anyone can learn to be more instrumentally rational by being trained in these skills by someone else. You can't learn to be more intelligent. (Though good sleep, exercise, and stimulant drugs all seem to help... at least according to IQ tests.)