IMO, being of a religious 'disposition' does NOT necessarily mean a person is stupid or unable to achieve anything in life eg be a good scientist/doctor/engineer or whatever.
And I would agree with Icebeing that being susceptible to religious feelings could be in part genetically coded and/or associated with our individual brain chemistry. So not as easy for some us to discard all religious notions.
If being religious makes some people more amibitous and/or more stable/more motivated/happier, then their beliefs might help them in their career, rather than hinder them? In other words, for them there might be an overall positive effect? But that would very much depend on what religious views they held!
However, I would still think less of people who 'believe' in something which I consider to be irrational, and I would tend to think think they could probably do even better/achieve even more without such 'beliefs'. (Assuming they were strong enough to kick away the 'crutch'.)
John Nash managed to work on his Nobel prize winning "Game Theory" while suffering from schizofrenia, but surely this doesn't mean he wouldn't have been able to achieve more without having schizofrenia? (The illness was more of a handicap I would assume?)
And so it is that religious people can be clever/do science, but maybe they could do even better without any religious limits to cloud their judgement?
Stephen Fry, (who is very clever), is open about his mental illness ... and about being an atheist! He probably considers his being bi-polar/a manic depressive to be part of who he is, and that his intelligence/personality is in some way inseparable from his brain disorder. So he might choose to keep it rather than to become someone else. But I think he would admit that during past psychotic episodes he was not thinking correctly ie he was deluded?
So I think that religion is similar to a psychotic delusion ... just milder and much longer lasting? (And encouraged by others with the same beliefs.) And while not totally debilitiating/negative, and sometimes an integral part of who the person is, being religious doesn't mean the person is not deluded, or that their beliefs are not totally imagined?