Ten or eleven years ago, I was at a party at a friend's house, a fellow by the name of Paul. Paul was a psychologist, and so it was no surprise that many of his guests were also psychologists. Upon introducing me to one fellow, Paul made a point of telling the guy that I had a degree in anthropology, focused in archaeology - I noticed Paul's grin, but didn't know quite what it meant at first.
The fellow to whom Paul had introduced me spent a few minutes conversing with me about various odds and ends, before he paused, and then asked "so, Mr. archaeology, what's the origin of religion?"
"well," I started to answer, "what aspect of religion are we talking about?"
He got a sadistic grin on his face and said "Oh no, you're not going to weasel out of this one! What is the origin of religion?"
I attempted to explain that religion is a rather complicated phenomenon, that different aspects of it likely had different origins, and that what we call religion today is simply the amalgamation of all of these aspects into the various belief systems that hover around our cultures, and therefore it wasn't possible to answer his question without breaking it down into several more specific questions. Again, I attempted to explain this, but he kept interrupting me and claiming that I was trying to "weasel out" of an answer.
Finally he stopped and said, in the most condescending tones I have ever heard come out of a mental health professional, "I know what the origin of religion is, and I can tell you."
"Yeah, it's pretty damn simple. See, one person with schizophrenia begins to hear voices, and decides that this must be god, so he goes on to tell someone else, who then tells another person that the first guy is hearing god, and it keeps going until the schizophrenic is a priest or a prophet, and everyone thinks that he's hearing god."
This sort of explanation for the origins of religion is common amongst the non-believers, but it is deeply flawed. I would have tried to point out the problems with it to this guy, but given his behavior up to that point, it was pretty clear that he wouldn't bother listening - he knew the answer (even if it was wrong) and plainly didn't want anyone trying to change his mind with inconvenient things like facts and evidence.
Still, his assertion had a number of problems. The first problem is simply that the notion of a god (or gods) has to come from somewhere before A) a schizophrenic can interpret hearing voices as being communications from a god, and B) people other than the schizophrenic can accept that the notion that a god is speaking through someone is a plausible thing. The psychologist Bruce Hood argues that a belief in spirits that animate the world is due to the way that our brains are designed, that (whe lacking materialistic explanations or sometimes in spite of materialistic explanations) we naturally see agency and therefore intelligence in the inanimate. This may be true, but if it is, then beliefs in gods and spirits would rise naturally from physically/psychologically normal people, no need for schizophrenia or any other forms of mental illness. So, if Bruce Hood is correct (and I think he makes some compelling arguments) this renders schizophrenia unnecessary for the formation of the supernatural beliefs that are the basis for most religious systems.
Another problem is that, on those occasions where it has been possible to examine shamans, which are probably our best analogy for the earliest clergy, for mental illness, the results have been decidedly mixed. Some studies have found that they exhibit signs of various mental illnesses, while others indicate that they are either no more likely to suffer mental illness than the average person, or even less likely do to careful screening by the elder shamans. What are we to make of this? Well, likely, this means that whether mental illness is a boon or a block to becoming a shaman is dependent on the culture in question, and we have absolutely no way of knowing which it was in the earliest of human cultures. So, while the first problem makes mental illness unnecessary for the origin of religion, this one makes it untestable, and makes the party-goers explanation an increasingly tenous "just-so" story.
Also, this fellow's explanation ignores the rather inconvenient fact that most religions provide a social code that is rather specific to the conditions of that society, and in most pre-literate societies these codes are maleable based on current conditions. Even if the belief in the supernatural were due to a mental illness, it wouldn't change the fact that by the time it becomes religion, complete with the trappings thereof, it has been heavily modified to suit the needs of society, or at least the desires of the society's louder voices.
So, the notion that religion began simply as a schizophrenic's ravings are untestable at best, and rather spurious. And understand, I am not an apologist for religion - if you click the tags on the side bar for "atheism" or "religion" you will quickly see my views on the subject - but that doesn't change the fact that I have a serious problem with people pushing their pet hypotheses without bothering to look at evidence. To be fair, this guy dealt with mental illness for a living, so it's no surprise that this is the first place that he would think to look, but that doesn't excuse the fact that it is also the only place that he was willing to look.