The thing about intelligence, testable or otherwise, where it relates to religion is that there used to be no real alternative to superstition to explain our existence. That didn't mean that everyone alive two hundred years ago was dumb, as they would certainly all fit somewhere on the modern intelligence spectrum, it means that religion dominated the discussion. There are still places today where that is true.
The founding fathers were part Christian and part deist, which would have been the closest thing to an atheist that the time allowed. I don't think the citizens of the time were any kinder to atheists than they are today and, certainly, politicians could not have made any definitive statements of doubt without serious repercussions, just as they must pander to theists today.
My point is, theism seems to be relative to something other than just intelligence, even today. The head of the human genome project was a theist but he was not dumb. If we are to identify some human component that is lacking in believers in superstition, it is more likely to be in their childhood credulity than their adult intellect. Those who never internalize the myths that they are taught by their parents are much less likely to have faith as an adult. I think it's also fair to consider the social, financial and emotional benefits that faith affords. If belief brings with it membership in a community that offers real dividends to its adherents, the rewards may ultimately "prove" god's favor.
Unfortunately, as with the Pentecostals I knew as a child, there are other benefits that aren't as nice but still serve the meme. For instance, knowing that you, above all your friends and neighbors, will be safe in god's bosom upon death, rather than cast into eternal fire and torture, even the painful sacrifices of faith, including ignoring the empirically true things that contradict religion, are considered to be spiritual investments and are "good".
So, faith has a cost and a reward, just like atheism. I guess what determines what you end up believing is the question, what do you need? If you need community more than you need to explain your existence without invoking magic, atheism is not for you. I am encouraged, though, at the atheist community that is being created in the US and elsewhere. Organizing around atheism, though, is hard to do without making it just another human division and no more useful, right or not, in the end game of unity.