The word "but" in the last sentence is a non-sequitur if there ever were one. Tegmark cosmology is not theism. Theism means Jehovah (etc). Yes, there are people who deny this, but those people are just trying to spread confusion in the hope of preventing unpleasant social conflicts. There is no legitimate sense in which Bostromian simulation arguments or Tegmarkian cosmological speculations could be said to be even vaguely memetically related to Jehovah-worship.
Isn't this - I'm sorry if that sounds harsh - arguing by a forceful say-so? Sure, if you constrain theism rhetorically to "Jehovah-worship", that practice doesn't sound very similar to the Bostromian arguments. But "Bostromian arguments/Tegmarkian speculations" and "the claim that a god created the universe" sound pretty much memetically related to me.
There is no need to pay any lip-service to ancient mistakes whose superficial resemblance to Tegmark (etc) is so slight that you would never notice it unless you were motivated to do so, or heard it from someone who was.
You're saying that e.g. "we are living in a simulation run by sentient beings" and "we are living in a universe created by a sentient being" are such wildly different ideas that there's only superficial resemblance between them, and even that resemblance is unlikely to be noticed by anyone just thinking about the issue, and is rather spread as a kind of a perverse meme.
Methinks thou dost protest too much.
The earliest time I can remember that anyone drew a very explicit connection between simulations and theism is in Stanislaw Lem's short story about Professor Corcoran. The book was originally published in 1971, when Bostrom was -2 years old. It's in the second volume of his Star Diaries; see "Further Reminiscences of Ijon Tichy: I" in this (probably pirated) scribd doc. I'd recommend it to anyone. Of course, it's very much possible that Lem wasn't the first to write up the idea.
Isn't this - I'm sorry if that sounds harsh - arguing by a forceful say-so? Sure, if you constrain theism rhetorically to "Jehovah-worship", that practice doesn't sound very similar to the Bostromian arguments. But "Bostromian arguments/Tegmarkian speculations" and "the claim that a god created the universe" sound pretty much memetically related to me.
See Religion's Claim to be Non-Disprovable for discussion of what religion is and how it arose. By "memetically related" I do not mean "memetically similar"...
Many folk here on LW take the simulation argument (in its more general forms) seriously. Many others take Singularitarianism1 seriously. Still others take Tegmark cosmology (and related big universe hypotheses) seriously. But then I see them proceed to self-describe as atheist (instead of omnitheist, theist, deist, having a predictive distribution over states of religious belief, et cetera), and many tend to be overtly dismissive of theism. Is this signalling cultural affiliation, an attempt to communicate a point estimate, or what?
I am especially confused that the theism/atheism debate is considered a closed question on Less Wrong. Eliezer's reformulations of the Problem of Evil in terms of Fun Theory provided a fresh look at theodicy, but I do not find those arguments conclusive. A look at Luke Muehlhauser's blog surprised me; the arguments against theism are just not nearly as convincing as I'd been brought up to believe2, nor nearly convincing enough to cause what I saw as massive overconfidence on the part of most atheists, aspiring rationalists or no.
It may be that theism is in the class of hypotheses that we have yet to develop a strong enough practice of rationality to handle, even if the hypothesis has non-negligible probability given our best understanding of the evidence. We are becoming adept at wielding Occam's razor, but it may be that we are still too foolhardy to wield Solomonoff's lightsaber Tegmark's Black Blade of Disaster without chopping off our own arm. The literature on cognitive biases gives us every reason to believe we are poorly equipped to reason about infinite cosmology, decision theory, the motives of superintelligences, or our place in the universe.
Due to these considerations, it is unclear if we should go ahead doing the equivalent of philosoraptorizing amidst these poorly asked questions so far outside the realm of science. This is not the sort of domain where one should tread if one is feeling insecure in one's sanity, and it is possible that no one should tread here. Human philosophers are probably not as good at philosophy as hypothetical Friendly AI philosophers (though we've seen in the cases of decision theory and utility functions that not everything can be left for the AI to solve). I don't want to stress your epistemology too much, since it's not like your immortal soul3 matters very much. Does it?
Added: By theism I do not mean the hypothesis that Jehovah created the universe. (Well, mostly.) I am talking about the possibility of agenty processes in general creating this universe, as opposed to impersonal math-like processes like cosmological natural selection.
Added: The answer to the question raised by the post is "Yes, theism is wrong, and we don't have good words for the thing that looks a lot like theism but has less unfortunate connotations, but we do know that calling it theism would be stupid." As to whether this universe gets most of its reality fluid from agenty creators... perhaps we will come back to that argument on a day with less distracting terminology on the table.
1 Of either the 'AI-go-FOOM' or 'someday we'll be able to do lots of brain emulations' variety.
2 I was never a theist, and only recently began to question some old assumptions about the likelihood of various Creators. This perhaps either lends credibility to my interest, or lends credibility to the idea that I'm insane.
3 Or the set of things that would have been translated to Archimedes by the Chronophone as the equivalent of an immortal soul (id est, whatever concept ends up being actually significant).