I only partly feel comfortable with calling the religious stupid. That in turn is only partly due to the fact that I was religious. I prefer not to walk around insulting myself in any sincerity, and I know I didn't get smarter as I became more secular. Religiosity in America and other parts of the world is cultural. A lot (I'd be alright with saying a majority) of people believe without properly understanding the foundation of their faith. That can be demonstrated by most religious individuals one is likely to encounter on a day-to-day basis. Many of the religious are also properly ignorant of basic science and replace an argument against a scientific principle with a remark based on their personal incredulity. Philosophy isn't a strong suit with the typical theist or spiritualist, this also can be observed easily when idiotic questions about the evolutionary origin of morality come up.
I do feel comfortable with calling some individual theists idiots if they fit the criteria (and a disproportionate number do), but not simply on account of their religiosity. I feel comfortable calling the theist ignorant, perhaps willfully so. As the title implies however, there are many who fall into either of the significant categories I've mentioned. These are the people who are taught from day one that a belief in some god is the pinnacle of their advancement as a human. I object strongly to that, as the reader may or may not have discovered. Frequently in such cases, critical thought is discouraged*. The individual may be brainwashed all too literally into their belief**. I hazard anyone who is eager to call the believer an idiot merely for their belief to think about everything that goes into keeping a religion alive.
Here's how it worked for me. I was born into a predominantly Christian family. From the time I exited the birth canal (if they waited that long), I was barraged with the baptist and young-earth mantra. I soon picked up on it and got saved when I was 3 years old at a VBS. I evidently don't remember much between then and when I was baptized at 5, a mistake which I may undo for personal and statistical reasons. For some reason my parents saw fit to teach critical thinking along with religion, perhaps in their naivety suspecting that I'd forever hold religion above criticism. I did the exact opposite, holding the opinion that keeping an idea from criticism is an insult to it's integrity. Of course I did so with the express preconception that my particular brand of theism would stand where others so reliably fell. I was ignorant...maybe a bit idiotic. At any rate, I took on the track of 2 years to dismantle the wall of brainwash-fueled ignorance. I'm now 17, going on 18 and an anti-theist. I was very ignorant, but intellect is the capacity rather than the knowledge.