I have started reading William James' The Varieties of Religious Experience. Published first in 1902, this collection of twenty lectures is considered to be a classic of the study of religion. It approaches the subject with a psychological as opposed to theological, historical, or apologetic angle, but appears to remain rather charitable towards religious beliefs.
This becomes clear already in the first lecture, much of which is spent assuring the believing reader that they have no reason to be offended by a psychological examination of religious experience.
James calls 'medical materialism' the idea that religion originated as the hallucinations and ravings of 'psychopaths' and 'degenerates' and can therefore be dismissed. (His words; see e.g. the interpretation of Saint Paul's vision of Jesus as the result of an epileptic seizure.) He argues that the value of a phenomenon, here religious truth claims, cannot be deduced from its origins; as an argumentum ad absurdum he points out that a scientific insight would be judged on its own merits even if the scientist who gained it was suffering from some mental disorder. By their fruits ye shall know them, not by their roots.
Well, fair enough, one might say. But while I cannot tell what the state of the discussion was around the year 1900, it seems as if this argument would miss the point of 'medical materialism' as it is applied today. Taking the position of an atheist, it is not the case that they attempt to answer the question of what to think of religious truth claims by looking at how they originated. They would most likely argue that that particular question has already been answered by applying the same criteria as James would (or at least the empirical one, see further down). They already take it as given that religious claims are largely false, and true only by lucky accident:
There is no evidence that there is something to us that lives on after death, and indeed the study of brain damages suggests that all there is to our personality is an emergent property of the physical. There is no evidence that the universe was created by a higher intelligence, and indeed it looks very much as if it wasn't. There is no evidence that the universe was created for our benefit, and indeed it looks very much as if it wasn't. There is no evidence that prayer works; and so on. There is also the small matter that hundreds of religions made and continue to make contradictory claims, meaning that only such a small percentage of them could be true as to be too close to zero percent to matter.
So given that background, the atheist now asks not what to think of a religious claim, but instead: How and why would people come up with something as wrong as that? And here hallucinations are a decent explanation for divine visions. That is why I feel that James' central argument in the first lecture misses its mark. But then again, he seemed to be more interested in reassuring religious readers than in criticising atheist ones anyway.
In this context it is also fascinating to examine what 'fruit' criteria James accepts as valid for judging spiritual and theological claims, now that he has rejected the 'root' criterion. He names three: immediate luminousness, philosophical reasonableness, and moral helpfulness.
Immediate luminousness is also described as based on 'our immediate feeling' upon being exposed to the claim. This seems rather oddly subjective and emotional, and at least in my eyes falls flat as a useful criterion.
Philosophical reasonableness is to be understood as based on how the claim relates to 'the rest of what we hold as true'. This is the most sensible of the three criteria, because that is also how we do it in science. If, for example, somebody presents us with the theories underlying homeopathy, such as water memory, we may consider in comparison what we believe we already understand about physics and chemistry. We then find that either large bodies of scientific knowledge supported by numerous experiments and empirical observations must all be utterly, mind-boggingly wrong, or that, alternatively, homeopathy must be nonsense. At this stage it should be easy to figure out which of the two options strains our credulity less.
Still, in the context of religious truth claims, this approach still appears unsatisfactory. How, after all, are any religious truth claims justified? If they are justified based on fitting into our body of scientific knowledge they are simply more scientific truth claims. If not, as of course they are, then each religion constitutes a network of beliefs that may (or may not) be internally consistent but that is completely unmoored from other such networks and from observable reality. The philosophical reasonableness criterion will have a Christian accept a vision of Jesus in heaven as true and reject a vision of the imminent death of the sun as false, and it will have a precolumbian Aztec reject the former as false and accept the latter as true, with exactly the same justification. How useful.
Finally, moral helpfulness suffers from exactly the same flaw as the previous does in a religious context. Unless the belief system is at some point anchored on empirical, observable reality, it is turtles all the way down.