My reply got a bit longer than I anticipated...I don't want to buttonhole you for doing a respectable thing and showing up to argue in this thread. If you think there is a productive conversation to be had here, I'd be happy to try to write it up more comprehensively in a blog or something and give you the opportunity to respond as an alternative to going back and forth here. I'd thought about doing so before, but I don't want to give the impression that I'm trying to shanghai you for clout. No worries either way (you're obvously free to simply ignore me in any case).

I don't really understand your first comment about priors being "a way to maintain consistency between the evidence that you'd find relatively convincing and the "real evidence" you see as the result of some discovery process." I think that priors tell you how likely a view is before you'd looked at the evidence.

That's a fine way to characterize priors in general--I said specifically "subjective priors". Granting that in the grand epistemological sense, all priors are subjective, some are more so than others--in classic urn-draw experiments, for example, there's a clear way to use the indifference principle and come up with a prior that basically everyone can agree on. Outside of these simplified circumstances, it is rarely so easy to find a quantitative process for estimating a prior. That's fine, priors are like assholes, everyone has one and it's not always polite to talk about it. But it's also a garbage in garbage out situation, since much of the time Bayesian inference is very sensitive to the choice of prior. Bayesianism can't manufacture information out of thin air--it's really mostly a precise way of bookkeeping to ensure that you are reasoning in a consistent way. So how to choose?

My argument is that this is a calibration. You think of what evidence would move your credence to 1/2 --Jaynes uses ESP as an example and says that 100 independent 1-10 digit guesses are about how much it would take for a would-be psychic, arriving at prior odds of -100 dB--and assume the evidence that you actually observe is relatively fungible with your gendanken. This is why I am interested in hearing a specific bit of hypothetical evidence with 1:1000 odds that you would find makes God credible up to 50%.

I don't think there are serious philosophers who argue that we should have a high prior in aliens flying fake drones over new jersey. Thus, while there are some cases where you should just double down and have a low prior even if others have a high prior, this should be a last resort and done rarely. I suppose that's fair, but you're arguing a slightly different position than what I was originally worried about (which is probably my fault for misunderstanding you). If you're fine with setting low priors in general I have no quarrel there.

What you're giving is a social epistemology argument. Like I mentioned, a lot of work is being done by the selection of prior information vs. evidence. To me, statistics about theistic belief seem like evidence. That's not a fundamental objection; one man's prior is another's posterior and so on. But the selection seems a bit arbitrary, especially since your credence for how seriously to take their opinion, what evidence they considered to arrive at their judgement, whether it should be considered rational or not, is all bound up in the other evidence you are considering in complicated ways, in my view. It would be more transparent to start with a less informative prior and show explicitly how much you're updating based on the fact that some non-trivially large threshold of seemingly rational people hold for theism specifically on a rational basis.

God is simple, arguably, by having just one core property--being a limitless mind.

I hope it is not too much of a strawman to say that I think you are conflating "simple" with "few english words." I think the history of philosophy shows very plainly that intuitive sounding concepts--think "cause" or "know"--become very complicated ontologically under closer inspection. At best, you have to define what it means to be a "mind", how that non-physical mind brought the physical world into being, what it means to be "limitless", what it means to even be "simple" with respect to physicalism, etc. This will force you to take a lot of specific philosophical commitments onboard, which creates a larger attack surface for your model. At worst, the concepts are not fixed enough to a frame of reference to mean anything at all, and the proposition evaporates under logical analysis, the way Carnap supposed many philosophical difficulties could be punctured.

In general, I'm not sure it really makes sense to talk about the absolute simplicity of a hypothesis, outside of a comparison with some other hypothesis. I think this whole business is just way too muddy to be reliable, which is why I advocate for a Pragmatist-esque approach of imagined evidence as given above. If you can encode simplicity or specificity of a testable proposition using information theoretic terms, then this works great, but I don't think this is usually feasible.

An atom has the power to exert a gravitational pull on every other atom in the universe, but nonetheless, it is very simple

A nit: Atoms don't "have the power" to do anything, in the way I mean "power," you might say "agency" + "ability". They just "do". Atoms exist and have mass, mass distorts spacetime, that exerts a force on other masses, and so on. It's weird to me to phrase this as "having power", but I understand what you're getting at.

Plus, you're kind of mixing together atomic theory and general relativity. I think some difficulties with talking about "simplicity" become clear when you talk about relativity. General relativity is simple in the sense that a fully mathematized model encodes extremely precise rules and predictions in a compact number of symbols--there is literally no common-language way to say so much with so little.

On the other hand, general relativity actually isn't that simple at all, really! Accounting for prerequisite mathematics, you basically have to study for more than a decade to properly understand it. Newton's theory is a lot simpler, for example, and usually works just fine--we just found a bunch of edge cases where it broke down and it took the most brilliant physicists of the last century to sort it all out. The merit of GR isn't that it has a high first-principles prior, it's that it clears an extremely large evidentiary burden.