You seem to be at the wrong meta level. (Er, sorry, social norms dictate that now I have to explain why I think that, but really I'm too lazy. Just interpret this comment as saying that you might want to look at the argument again to make sure you're appreciating its meta-level points.)
So, by "meta level" I assume Will just means "the rest of Plantinga's argument" which is that if beliefs aren't reliable than there is no reason to believe naturalism (not a reliable belief).
The key of course is the phrase "perfectly reliable" which Gwern uses and Plantinga does not. Plantinga admits that beliefs are not perfectly reliable- when he says reliable he means something like "the vast majority of the time". He has a specific argument that a belief causing adaptive behavior is not an indicator of its truth....
A friend recently asked how strongly I believe that my deconversion from Christianity was not a mistake. Here's my response, and for those of you who are not Christians, I'm just wondering what numbers you would give:
"There is a part of me that wants to say the chance is far less than 1 percent. But when I consider what 1% must mean about my ability to follow complex arguments and base my judgement on the right premises, it seems absurd to say that.
Trying to honestly estimate the chance that I'm wrong about the Bible being generally reliable is a fascinating exercise... I know the number is low, but I'm not sure how low.
Today I would give myself a 1 in 20 chance of being wrong. If I were to consider the arguments of 20 other groups similar to Christian theologians, I would probably misunderstand them at least 1 time in 20. After talking with 20 groups that have a very different worldview, I might think they are all are mistaken, but once in a while, maybe 5% of the time, it would actually be me.
Wow, 5%!?! If I convert that into "There is a 5% probability that the God of the Bible exists and will send me to hell", I feel scared. But I know how to cheer myself up: I just say, "No way, the chance I'll end up in hell MUST be less than 5%. After all, the God of the Bible is CLEARLY just a big, mean alpha-monkey and... [rehearse all the atheistic arguments here]".
This back-and-forth from certainty to uncertainty makes me feel like I'm doing something seriously wrong.
So what about you? What chance do you place on some variant of Christianity turning up to be true, and what chance do you think a god of some sort exists?"
Numbers please.