I am just in awe that people fall for stuff like that...
I mean, it would be bad enough if you believed it the very first time some religious nut made such claims but people fall for this shit over and over and over and over and over and over and over and over...
EDIT: I want to add that I believe in a much broader definition of mentally disabled than what is standard among medical professionals... It seems that the main stream view is that you are only dumb to the point of being disabled if you are below certain statistical points (often two or three standard deviations below average). But personally I define mental disabilities as being incapable of rationality. Independent of how many people are also that dumb.
For example... I remember in my economics senior seminar the professor challenged the class to distinguish normative from positive statements. Positive statements are testable/objective, while normative are not. For me this was a very obvious, easy task, but one guy in the class really struggled with it... He even tried to say that a statement by Carl Marx had to be normative just because Marx said a lot of other normative things.
In my view, that guy is mentally disabled because I think that is a very, VERY basic reasoning skill. I can think of so many times I was called stupid for expressing a totally subjective opinion. I'm amazed at people who just don't know the difference.
By the same token, I think you are mentally disabled (or insane depending on how it goes on in your head) if you can't distinguish between something that you just think sounds right, and something you actually deduced or observed. People who believed the rapture was going to happen to the point where they made major life decisions on it (like spending their life savings) have this disability.