This is a difficult subject to understand nonwithstanding my admiration for much of Richard Dawkins work. I'll try to explain this as I see it the best i can, but will not do so well because you really need a book for this..
ok, so we must clarify what we mean by reality and by default delusion. However, that would take too long and involves so many nuances that I cannot go into that here, so here's a shortcut and am sorry for anything that's assumed or unclear:
So, in the sense we are talking about here, we all have delusions and would all be to some degree "mentally ill". However, like "crime", these terms are human constructs and are so defined as legal constructs, not "reality". The "reality" as I see it is that our brains are designed to function with fictions from the get-go. It is a function of our experential mechanism that allows us then to sort what is perceived as being fact or fiction. However, it is also a function of that same system that can indicate a fact as fiction (this, still relative to us!) and fiction as fact. Examine the rain dance.
Science is a methodology designed to cut through this experential haze and marshall our experiences into better forms that enable more sophisticated analysis than what our hapharzard/random experential systems receive on their own. Still, science is not perfect and we can obtain false positives and negatives.
So what we are left with is a system that perforce shoves us into believing in false things, ghosts, demons, rain dances, broken mirrors and bad luck etc. It is in our nature and sans science runs rampant (though even with science it is still wanting and there is still so much outside of the purview of science that there is a lot of illusions left for us to experience). For more on this..see behaviourism and learning - both classical and operational.
Language as it relates to our consciouness, for example, begs gods and demons. Why? Because we have reflection and so can talk to ourselves (probably reflection preceded language). This means we can talk to many people in our head. We are also polymorphously perverse. Something else a result of our consciousness (at least as it relates to our conscious "choices"; animal love of bizarre objects can be induced as well - though for different reasons and effects) which means we can project anything in our consciousness and subconscious to other things.
The result here is that you can have a conversation with yourself or make up a conversation with someone or something or some God in your head. Children are excellent at this with stuffed animals. This is one of our human superpowers if you will - a "side effect" of consciousness.
So, when ideas are processed that we call irrational or delusional, there is essentially no difference in processing that information in our brains than what we call rational. The only difference occurs in deciding either individually or collectively that certain beliefs that are not ours are irrational or perhaps better stated: something other than rational.
"Delusional" then becomes a social construct. A schizophrenic in a tribe is honoured and can lead and heal the tribe. In our culture they are marginalized, hospitalized and considered nuts. Note, I am not making a judgement on either cultures correctness, or morality here, just trying to set a playing field (defining terms) for that discussion if it is desired.
So, religious beliefs are not a mental disorder any more than non religious beliefs are a mental disorder. They are only such from a human level, a construct such as crime which is human made and hence has no "reality" beyond what society decides at a particular time.
Speaking of shamans and suhc, note that native american tribes considered the "white man" to be mentally disordered because he claimed to "own" the land and because of his acquisitiveness in general. Was that a mental disorder? Only recently has western people (outside of some religious sects) begun to question materialism and that questioning is still no where near the level the native americans thought about it.
Aagin, I have left out a ton so there's a lot of assumption in my discussion but anyway its .02..