Actually, I think that the charitable assumption is that at least some religious/spiritual people may have been exposed to a different set of evidence than you have and that, even if not ultimately correct, the hypothesis "yes, there is a supernatural realm that interacts somehow with the material world" may actually be be most rational explanation for the evidence they have available from their lives, their experience of the world.

I think that starting from any assumption that excludes that possibility is not only factually wrong but also judgmental and laden with prejudice. It suffers from pretty much all the same problems that "everyone is spiritual in their own way" does, with the admittedly significant exception that (outside of the scientific community) atheism is the minority in our culture.

I don't know about "almost any other context", but in the scientific community for sure. Of course, the scientific community has agreed on the a priori assumption that rationalism and empirical materialism are the only philosophical schools of thought with any validity. This may, ultimately, turn out to be accurate, but I don't think it's fair to say that no reasonable, thinking person could question that assumption or arrive at a different conclusion.

Certainly it's an option to conclude that my perception of the outside world is profoundly flawed. It's legitimately possible that my brain is being kept alive in a jar and fed input stimuli through a neural interface, or my body is generating heat energy for the machines running the Matrix, or there was never a body at all and my entire "self" is a program running in some sort of emulator. However, barring cases where the input of my senses and memory is actually internally inconsistent or I have a particular reason to expect my perception to be altered/unreliable (eg I remember eating this Guatemalan suicide pepper, and then there was a coyote telling me to find my soul-mate), it seems like a reasonable starting point for me to provisionally accept my perception of the world as being a reasonably accurate reflection of an external reality that actually exists. If my perception and my "belief system" (ie my best current understanding of how the universe works) are in conflict, then both are subject to question.

For what it's worth, while I fully accept that this may not challenge your conviction that taking delusion as the default assumption is the more rational approach, I would like to clarify that when I refer to evidence for a spiritual realm, I do mean something much broader than the "rays of light and an overwhelming sense that there's something more out there" spiritual experience thing. I'm including things like predictive power (if someone repeatedly tells you "God told me this (apparently up-to-chance) situation will play out thus-and-so" and it repeatedly turns out to be accurate, well, delusions are not generally known for being accurate predictors of real-world outcomes. Also interpersonal experiences, such as having otherwise-reliable people with no history of hallucinations report having seen/experienced the same thing. I think that the belief in love in a relationship sets an interesting example, too: there are certain people who I know with tremendous confidence love me. My knowledge on this point is based on a body of evidence that's very similar to what I describe above, which is not really transobservable or reproducible. My experience of being in a given relationship is by definition a very different experience than that of an observer looking on the relationship. Certainly it's technically possible that one or more of these people is an amazing actor with ulterior motives and the patience of an Ent, but the evidence available to me from my lifetime of experience strongly suggests that their love is a real part of the real world. The impossibility of conveying that knowledge to somebody who has not had similar experiences does not, to my mind, undermine its reality--especially given that I can compare notes with a significant number of other people who have had similar experiences and arrived independently at the same conclusion I have. It most certainly does open the door to accepting that their unbelief is the most rational response to the evidence available to them, though.

And finally, suppose as a variation on your "if Jesus returned to Earth next week" thought-experiment that Jesus appeared to a room-full of a couple of dozen people, did a few miracles, and told them "I have my reasons for not appearing to the world at large yet, you'll have to trust me on this". If, after verifying that everyone had not eaten from the same dish or shared wine etc, these couple of dozen people found no external factors to suggest mass hallucination and all confirmed the same experience, well, would it be more rational for them to conclude that it had happened, or that the whole group started spontaneously hallucinating at the same time for no reason and all had exactly the same trip?

Whoo! First off, I'd like to say that I hope you're enjoying this as much as I am.
Also, Sabs, sorry for hijacking your post (uh, at least it's a week-old post...?) I'd have no problem just letting this drop if you'd prefer we don't have this discussion in your space.
So, that said, here goes.

I think that your examples of "transobservabiility forms the foundation of how we navigate the world" vary significantly in how effectively they support your position. "What kind of car should I buy?" for sure--that's generally very much based on objective factors like cost, fuel economy, trunk space, etc. "How is my health?" though, can be overwhelmingly determined by something like chronic pain or fatigue with no apparent cause. I can't observe, feel, or measure your pain; I am forced to choose whether to take your word for it (aided, perhaps, by observing that you behave like someone who is in pain) or decide that you are lying or deluded. I have a friend who spent decades being confidently assured by the medical establishment that she was just lazy and that chronic fatigue was a junk diagnosis, before fibromyalgia was widely recognized as a real condition. She had access to evidence that the doctors chose to dismiss as delusion because it wasn't transobservable and they couldn't link it to a physical cause. Similarly, the answer to "is this or that romantic partner right for me" can be to a significant extent determined by things like "how does it make me feel to be around this person?" While you could in principle map the neural activity and chemical changes that correlate to the feeling, the feeling is not transobservable. The feeling, in and of itself, is a piece of real, relevant evidence that is available only to the person experiencing it.

As for "I would simply include hearing voices, feeling the hand of God, seeing an angel, etc., in the same category as talking to a coyote", That's certainly a reasonable position to take. ;-) What about, say, attributing consciousness to the poorly understood, deeply interconnected, and mind-bendingly complex system that is the entire physical universe? Without tackling the question of whether it's true or not, would you be willing to grant that the fact of the universe having given rise to life and consciousness could rationally be taken as evidence that the universe as a whole might possess the characteristics of life and consciousness?

Oh, and just for the record, vivid hallucinations with no external stimulus being infinitely more common than talking coyotes is not actually established; that's pretty much the heart of what we're discussing, isn't it? Reports--from people with no (other) symptoms of mental illness, no history of drug use, etc.--of seeing angels or feeling the presence of God are widespread and persistent. Now, your starting assumption (and it's certainly a reasonable, defensible starting assumption) is that each of these reports is an example of a vivid hallucination, because you have already concluded that angels and God do not exist. Another possible, rational interpretation is that these reports constitute evidence (weak evidence, perhaps) for the existence of the beings being reported. It's entirely conceivable that some people imagine these things and some people genuinely witness them, in much the same way that some people have paranoid delusions of being persecuted by powerful agencies and some people genuinely are.

"This would bridge the same gap that having Jesus/God in an interview room would." I think we're in full agreement on this point.

Cool, cool. Glad to hear it.

What I would argue is that the feeling doesn’t need to be transobservable in order to convey justifiable belief. The “force” of gravity, for example, isn’t directly observable and yet we seem to believe in it, simply as a result of the preponderance and regularity of its effects.

Ah, but here you're sidestepping the everyday examples that you yourself brought up and reframing the conversation in the rationalism and empirical materialism of experimental science. I maintain that the answers to everyday questions that are fundamental to how we navigate the world, such as "how is my health" and "is this or that romantic partner right for me" can hinge crucially on evidence that is neither transobservable nor reproducible, and yet both real and vitally relevant. Chronic pain exists regardless of "conveying justifiable belief". The pain is, in and of itself, a determining factor in answering the question "how is my health?" and is neither transobservable nor reproducible. Whether my doctor (or any arbitrary skeptic) is satisfied as to the existence of my pain through observation of its indirect effects or not, (and often the answer is "not", especially for patients who endure pain stoically) my health still suffers from the the simple fact that I hurt. (Disclaimer: I, myself, thankfully do not suffer from chronic pain. I am using it as an example only.)

Sure, there are lots of examples of people, individually and en masse, believing things that end up being firmly disproved. One of my personal favourite examples is the whole perpetual energy schtick. And yes, eyewitness accounts can certainly be outweighed by stronger, contradictory evidence, as in your example of the "chi master". Eyewitness accounts are not proof, but they are most assuredly evidence. If a generally truthful and reliable friend comes over to my house and comments that there was a black cat sitting at the end of my street when they walked by, I'll be inclined to believe that it's true even if I've never noticed a black cat on my street, and even if I go look and don't see one now (cats do come and go for reasons of their own, after all). If various friends and neighbors make comments about this black cat that I have never seen, then I'll be that much more likely to assume that it exists and I've just never happened to cross paths with it. Because my prior experience of the world indicates that there is such a thing as a black cat and that a residential street is a plausible place to find one, my starting assumption is that I have no reason not to believe these reports. If, on the other hand, this same parade of people commented that they had seen, say, a live hippogriff at the end of my street--well, in that case my initial assumption would be that they were hallucinating or being pranked. Why? Is a hippogriff that I've never seen somehow a more or less transobservable phenomenon than a black cat that I've never seen? Nope. But I know with pretty good confidence that black cats exist and hippogriffs don't. And yes, you're right, to be precise I should have said that you have already concluded that these particular phenomena (the perceptible presence of God, and visitation by visible angels) do not exist. The fact remains that the reason why you're assuming all accounts of angels, the hand of God, talking coyotes and so on are hallucinations is not because all unconfirmed eyewitness accounts of transient phenomena are assumed to be hallucinatory. If that were the case, then every account of a fog bank that burned off in the morning sun or a passing butterfly would similarly be dismissed as hallucination. No, these particular accounts are assumed to be hallucinatory because you have a prior belief that the things described are impossible or implausible--a belief which is, to the best of your understanding, supported by stronger evidence than that provided by eyewitness testimony to the contrary.

Among people whose life experiences are similar enough that we agree on the existence of genuine love, sure, we say things like "it's clear she loves you very much" without much problem. I don't know about you, but I have also known people whose personal experiences with everything they were ever told was love were characterized by a pattern of people being nice, paying attention, claiming to love them, gaining their trust, and eventually it comes out that it was all because they wanted to get something out of the situation (sex, popularity, money, whatever). Based on the body of evidence that person has available to them, the rational conclusion that love is fake and if you believe somebody actually loves you then you're a sucker. This person is just as sure that I'm deluded to believe somebody genuinely loves me as you are that I'm deluded to believe there might be a spiritual reality. My conclusion that love can be real is based on my personal experience of every relationship I've ever had. Your own experience may have also led you to the conclusion that genuine love can exist, but you are working from your own, non-transobservable body of evidence. Nothing you can say will convey the knowledge of genuine love to that person, because according to the very real body of evidence offered by their own life experience, you're clearly deluded and any "real" loving relationship you hold up as an example simply hasn't yet reached the "it comes out that it was all because they wanted to get something out of the situation" phase.

"In any event like this, a case for mass hallucination (which occur) would still be better than the case for miracles (which presumably do not)." Okay. Good to know that reference point. And from the outside, I most definitely agree. A couple of dozen people reporting such a thing... easily dismissed. If I were one of the people in that room, I'm not sure which would yield first, "I know what I experienced and it was verified by dozens of other people" or "I know Jesus doesn't turn up in a Greyhound station and show off a few miracles." I think that for myself, my willingness to question the veracity of my senses and my willingness to question the things I know to be impossible might be pretty close to equal, lacking some external factor to point to hallucination.

Okay, I'm game for a foray into the romantic and alluring world of the digestibility of lactose in rats!

The key point that I take from your example is that both scientists have rationally and correctly evaluated the evidence available to them and arrived at the most rational answer, which turns out to be wrong due to factors they had no way of knowing. Heck, even without the switcharoo at the end: if we take your two scientists with the two groups of rats and we take as a given that neither of the scientists has any way to know that the rats are genetically different and each one assumes that the rats she's working with are representative of rat-kind in general, then at the end you ask the question "can rats digest lactose?" Each one will give you a different answer. The key thing here is that neither of them is hallucinating or believes what they believe because of some social or psychological need. Each of them has rigorously and rationally evaluated the evidence available to her and arrived at the most reasonable answer.

As for the simmer of Pyrex in the centrifuge, while admittedly poetic, I have to regard that as a red herring. The point on which your example fails as a parallel to the "does genuine love exist" example is not on a lack of emotional involvement, but the fact that the non-conveyable sensory fullness of the experience of watching rat urine spin in a centrifuge is utterly irrelevant to the experiment and its outcome.

When the hypothesis being tested is "genuine selfless love can be found among human beings", on the other hand, the body of evidence at your disposal critically includes your lifetime of experiences with how people who have claimed to love you (and acted as though they loved you) ended up either sticking with you through thick and thin or betraying their ulterior motives and taking advantage of you. If one person's experience is consistently characterized by people lying, maintaining the facade (perhaps for years), and ultimately it comes out that it was for their own advantage, and another person's experience includes one or more examples of somebody demonstrating loyalty when they have nothing to gain and everything to lose... well, these two people can rationally examine the evidence at their disposal and arrive at two very different conclusions. Further, each of these people can reasonably and rationally conclude, based on the body of evidence at their disposal, that the other person is deluded/deceived and anything they have to say on the subject is highly suspect. The real and relevant body of evidence that rationally dictates the acceptance or rejection of the hypothesis in this case is non-transobservable. They can't trade rats and re-run the experiment. The shimmer of Pyrex in the centrifuge is as irrelevant as the sensory fullness of the smell of a particular lover's skin.

The way we evaluate the credibility of these beliefs...

In cases where no justificatory evidence is accessible, I maintain that a rational person is entirely justified in maintaining an open and unapologetic skepticism.

Absolutely. I have not, at any point, questioned your justification in maintaining that the beliefs are not credible. I am entirely willing to accept that you have, based on proper interpretation of the fullness of the evidence available to you, concluded that any belief in in a religious or spiritual reality is a mistaken belief. I fully support this open and unapologetic skepticism.

However, when you take the step from evaluating the credibility of the beliefs to concluding--in the face of unrefuted examples to the contrary--that you, personally, in the real, present world, have sufficient access to the dataset on which another person is basing their conclusions to know with certainty that there's no way they can have rationally arrived at a wrong conclusion based on having properly interpreted a misleading dataset (like the scientists whose rats have been secretly switched), that's not skepticism. That's arrogance and prejudice.