If someone holds a belief, and you disagree, then they are demeaning themselves?
Frequently - not because of my disagreement, because some beliefs demean the people who hold them. Ugly, pernicious, noxious beliefs such as racism and homophobia; ill-thought-out, gullibly accepted, rarely (if ever) questioned and tested beliefs that stand in contravention of all human experience such as theism.
You demean someone by impugning their intelligence, and that is just nasty.
No, because frequently religious belief has little or even nothing at all to do with intelligence
per se. Some extremely intelligent people (intelligent otherwise, I mean) have been and are religious believers; as I see it this is because their religious beliefs have nothing to do with their intelligence - they simply allow them to bypass their critical faculties, thus escaping the same sort of hard, sceptical analytical thought that they would bring to bear on anything (or everything) else, usually for emotional reasons. What else explains the uncommon phenomenon of the religious scientist? As someone (Swift, it may have been ... I'm not sure) said, you can't reason a man out of what he was never reasoned into in the first place.
Einstein you ain't!
I don't claim to be ... but then, one thing I share with the good Professor (or wee Albert as Gonners like to call him) is that belief in a personal god is childish