I have told this to both religious people and atheists in the past, and both have actually given me quizzical looks. The religious person because they assume that non-believers think believers must be nuts and atheists because, frankly, many of them think believers are nuts (though clearly not all; many atheists used to be devoutly religious and do not think that their losing religious convictions involved becoming more sane.) Therefore, let me explain myself lest I get in trouble with both groups.
The DSM defines "delusion" as "false beliefs based on incorrect inference about external reality that persist despite the evidence to the contrary and these beliefs are not ordinarily accepted by other members of the person's culture or subculture." They further distinguish between bizarre and non-bizzare delusions. Examples of non-bizzare delusions are "being followed, being loved, having an infection, and being deceived by one's spouse" (presumably, in the case when none of those things are true and there is demonstrable evidence to the contrary.) This is to be seperated from bizarre delusions, which are not derived from daily life experiences and which are signs of more severe medical conditions such as schizophrenia. What I would like to discuss here is whether or not non-believers should say that religious people are delusional.
Certainly, some religious people are delusional. On that level, I think even most devoutly religious people would agree with me. If a religious person were to claim that there were no religious people that were delusional at all, that would ignore the reality of mental disorder in people coming from all walks of life. Mental disturbance does not choose only those from any particular belief system or cultural background. It's something that we can all be subject to, and all be potential victims of.
There is empirical evidence from the abnormal psychology literature to support the claim that often, in schizophrenic people, hallucinations include religious iconography. Schizophrenia is often more severe amongst religious patients, and while it is tempting to hypothesize a causal relation here, we must tread very carefully.
But even amongst scientific crackpots, there is a plethora of mentally unbalanced individuals who hold strange ideas about, say, physics. Consider Bayard Pfundtner Peakes, who killed a young girl working at the offices of the American Physical Society, or the man that used to send my undergraduate department angry letters (and who once sent me a list of the people to whom he wanted to do harm.) Both of these individuals are documented as being schizophrenic and both held intensely strange ideas about science. There is no reason to conclude from the fact that schizophrenics often have religiously themed hallucinations that religion itself is to be disparaged, any more so than we should disparage physics for the crackpots that appear there (even the crackpots with severe mental disorders should not cause us to disparage physics). There might be still other reasons to disparage religion -- that has yet to be seen and I certainly know many people who would argue that there are many things to disparage about religion. Nonetheless, the question I would like to address here is whether psychologically normal people who hold religious convictions, and that is more than likely the majority of people holding religious convictions, should be called delusional by those who disagree with the belief-content of the believers.
First, I want to establish that there really is no indication that religious convictions lead inexorably to severe mental illnesses such as schizophrenia. In fact, I would argue that there is no clear statistical relationship between the two. Around 3% of people regularly hear voices in their heads. Since auditory hallucinations are known to be more common than visual hallucinations, and since, to first approximation anyway, religious conviction and the appearance of auditory hallucinations are independent variables, we can surmise that around 0.81*0.03 = 2.43 percent of American Christians suffer from hallucinations. That's an awfully low number. Only about 1% of people in the US suffer from schizophrenia, so the chances of being schizophrenic and being a Christian are roughly 0.81 percent (that is, neglecting correlations between the two variables.) Since I neglected correlations in both of these cases, these are really just lower bounds. Nonetheless, the actual figures cannot be much higher. Even if all schizophrenic people were Christians, there are still are more psychologically normal Christians than psychologically abnormal ones. Clearly, whether we regard religious people as delusional or not, most of them are not schizophrenic. Whether or not most schizophrenics are religious, and how religion functions in the treatment of schizophrenics, is an entirely different question and one for which there has been done some empirical work.
Next, I would like to investigate the claim that psychologically normal people who possess religious convictions are necessarily delusional. It was C.S. Lewis who first popularised his famous tri-lemma for the lay Christian audience, claiming that Jesus must have been either delusional, a liar, or the son of God. I think the notion that non-believers should address believers as delusional stems from this same, or similar, line of apologetics. The rhetoric would proceed that Christians are either delusional, liars, or that the content of their beliefs has real ontological significance (i.e. either liars, delusional, or God exists).
There's an excellent scene in the movie Contact in which Matthew Mcconaughey's character, Palmer Joss, tells Jodie Foster's character, Ellie Arroway, that he couldn't, in good conscience, send someone to represent mankind who believed that most people on Earth are delusional. The fact that this is stated so matter-of-factly, and that Arroway doesn't have a clear rebuttal, means that the audience is supposed to implicitly agree. Yes, the audience is supposed to think, Arroway does think most people are delusional due to their religious beliefs and this is problematic. Of course, this issue is resolved by the end of the film, at which point Foster's character finds something to have faith in (albeit, not God, but the message -- that what Daniel Dennet calls belief in belief is important -- is clear.)
Yet the most common retort to Lewis' trilemma is that it is a false dichotomy. Clearly, the first part of the DSM's definition of delusion -- that delusional people are those who possess "false beliefs based on incorrect inference about external reality that persist despite the evidence to the contrary" -- describes the relationship many psychologically normal people have to their beliefs. Yet the second part of the definition clearly states that to be delusional, "these beliefs are [such that they are] not ordinarily accepted by other members of the person's culture or subculture." For most religious people, that second condition does not hold.
One can simply be mistaken about an issue, or simply reason poorly due to one's own emotional biases or other factors, and not be delusional. It would be grossly poor rhetoric to simply state that everyone who one disagrees with is delusional. This would be highly dismissive and would likely represent an ad hominem type fallacy. This is partly fixed by adding the caveat about still holding the belief despite evidence to the contrary, but there is no clear and unambigious level of evidence at which we should be justified in discarding previously strongly held beliefs. At what point we are willing to accept or reject the truth or falsehood of a given proposition has much to do with our previous cultural leanings, educational level, and socio-economic status. Even in the sciences, statistical acceptance level is something which changes from one science to another, or even within sub-disciplines. In particle physics, practitioners are willing to anounce a discovery when it is significant to the 5 sigma level. In many social sciences, only 1 or 2 sigma is deemed necessary. Even the identification of what counts as a theory, and can therefore be accepted as explication about the world changes from one discipline to another. What social and biological scientists are often willing to accept as theories would never even begin to pass muster in the physical sciences. Of course, there are strong statistical justifications for the acceptance criteria (or at least there should be; whether or not there always is, in practice, is a different story) and there are other kinds of justification to be had for the diverse forms of scientific theory between disciplines. The degree to which these things are arbitrary, and to which culture and historical accident affects them, is something that those who scientifically and philosophically investigate scientific disciplines are currently trying to establish (as in the fields of science studies, philosophy of science, and sociology of science.) That culture and historical accident affects them to at least some degree is definitely clear.
Just as in the case of any Gestalt image, I believe that we can either see some internal state as indicating the existence of a god or as something else altogether based upon such factors as our respective cultural backgrounds. Ask yourself whether the following image is that of a rabbit or a duck?
Likewise, is the following an old woman or a young woman?
Thomas Kuhn first brought up the example of gestalt images in his Structure of Scientific Revolutions in order to explain why people, living under different conceptual paradigms, are willing to accept different explanations about the same data. Physicists living prior to the advent of quantum mechanics were likely to interpret all available data in terms of classical mechanics. Afterwards, at least some of that data was interpreted differently. It was Kuhn's contention that someone from before the revolution would not even be able to talk to someone living afterwards. The two world views are simply incommensurable, despite sharing a large portion of the same amount of data. I do not think that the case is as severe as Kuhn contends, and I definitely do not agree with some (probably most) of the post-modernist critiques of science that followed in Kuhn's wake. Nonetheless, there is an important lesson here for how I think believers and non-believers should view each other.
Not being able to directly interrogate the minds of other people, I cannot definitively conclude that both believers and non-believers experience the same mental states but attribute different explanations to them. However, Ludwig Wittgenstein, writing in his Tractatus, had a splendidly good example to drive this point home. This example is usually used to explain the problem of other minds -- this is the problem of explaining how I can possibly know the mental content (or even if there is mental content) of other individuals.
Suppose that we organize people together into a room, and we give each of them a box. Inside each box is a beetle. No person is allowed to look in any one else's box. You now allow them to open their respective boxes and each examine the contents. They all exclaim that they have beetles inside their boxes. Yet what does that word possibly convey one to the other? Certainly, they cannot compare the contents of their boxes directly (by construction). Any given individual might be using language in a strange, non-standard way, disallowing him or her to communicate the box contents. Likewise, if they tried to give a description of the contents of the box, we still should not believe him or her since they might still be using language improperly. That defeats any possible process that we might have of interrogating the person so as to discover the content of the box.
Likewise, I cannot directly read the minds of religious people. I do not know what phenomenal experience is occuring when they claim to be in contact with God. Of course, since I do not have direct access to their mental states, they cannot expect me to simply believe in God based on their say-so. Even if I experienced God, whatever that might mean, I would still doubt His existence. This is because I realise that there are all kinds of alternative explanations related to the functioning, or malfunctioning, of the human mind and I would need to rule all of those out before I was capable of accepting the belief. People who are from other cultural backgrounds or intellectual tastes likely have different criteria for when they are willing to accept or reject beliefs, and will therefore be either more or less likely to come to religious conclusions based on those same mental states.
Nonetheless, I think all people, regardless of cultural background or prior state of belief, are capable of spiritual feelings, numinous, profound ecstasy, or rapture. I strongly suspect that these kinds of emotions -- or some variant thereof -- is responsible for the Christian conception of feeling their God's presence. This is certainly consistent with what I know about religious experience and it is consistent with other facts that I know about the world. It doesn't diminish the experience or mean that the emotions which are felt are somehow less than real. Certainly, as William James once argued, religious experience is a very real phenomenon and quite worthy of empirical investigation. But the reality and the profound nature of an experience does not, alone, unambiguously indicate which ontological stance we should take with respect to the world.
As a last point, if my theory about religious feelings is correct, then it could serve to facilitate a better understanding not just between believer and non-believer, but between believer and believer. I believe that, if true and accepted by more people, it could foster increased religious tolerance. If the religious feelings are truly the same between all of these religions, but only the attributions are different, then believers of all different backgrounds should have a profound and honest respect for each other. After all, that would mean that their respective beliefs are held for the same reasons and that, to each respective individual, those feelings are just as sacred as those of the others. On this ground, I sympathise with the Unitarian Universalist notion that all religions are simply different ways of knowing the same sorts of fundamental truths. While I strongly disagree with the notion that all belief systems can be simultaneously true (and would directly deny the conceptual relativism this implies), I find that there should be a common understanding that these different religions are simply motivated by the same emotional underpinnings seen through different lenses.