I don't think belief has to much to do with intelligence. I consider myself below average intelligence but, a true nonbeliever. Being raised in a Catholic family, Indoctrinated in Catholic school, being an alter boy, I was able to finally understand that it was an irrational belief.
What I can't understand are how vary intelligent people can believe that a supreme being created the universe and cares about your every thought and deed.
Your point about mental capacity made me think about my severely mentally retarded brother who nether believes nor disbelieves.