I would have to agree with jam. You would have to differentiate between born-ins and converts to get an accurate analysis. Obviously, the majority of those who convert to a religion such as Jehovah's Witnesses would have to be sub-par when it comes to critical thinking skills. The ones who are born into the religion and are brainwashed from birth on have an existential crisis waiting for them as they approach adulthood if they do possess an above-average intelligence.
However, if you give any weight to the theological perspective, this is irrelevant to you. After all, he has "hidden these things from the wise and intellectual ones and has revealed them to babes."