Considering intellligence quotients only, I don't think it is an empirical fact that secularists are necessarily more intelligent than religionists. There are plenty of not-so-bright secularists and quite bright believers (using "bright" in its older meaning, not the recently popularized one), as we all know.

However, when I try to come up with an explanation of how smart people can be believers, the best I can do is to assume that, when it comes to religious matters, they in effect disable their reasoning powers, which work perfectly well otherwise. With these powers disabled, it's not surprising that they come to what we consider the wrong conclusions.

Why do they disable their reason? Perhaps it has to do with fear of death, or the comfortable feeling they get from holding onto beliefs cherished since childhood. In any case, I think it is perfectly possible for them to be wrong, but not stupid, and we secularists should try to get out of the habit of calling them stupid.

But you use the word "ignorant," which is not the same thing as stupid, of course. "Ignorant" means not knowing certain facts. Perhaps some believers are ignorant of some facts which would disprove their beliefs, or at least count as strong evidence against them. More likely, they simply "ignore" them, in the sense that they don't pay attention to anything that would tend to refute their beliefs. But I would not want to argue that all believers are ignorant in either of these senses. There are some people who are quite informed and quite intelligent, as well as quite aware of arguments against religion, but simply put their religious beliefs into a protected mental area where nothing is allowed to contradict them.

One thing I can say for sure is that religion seems to deserve about as much respect as a belief in astrology, but I've been taught all my life to give it a greater portion.  I can only chalk this up to the fact that those who are offended by a lack of respect for religious beliefs tend to carry a much bigger stick.

At least, that's how I feel right now.  Sometimes I'm highly respectful of certain religious beliefs and practices, so long as those beliefs and practices seem harmless and positive enough to my mind.