Dan then justifies the premises:
Surely it's analytic that God is a necessary being (if God exists at all). And it's equally obvious that God is possible; after all, God is conceivable. Therefore, God exists.
Now, this is a terrible argument for God's existence. We could just as well apply it to any conceivably necessary entity. For example: it seems possible that there could be a necessary blob, therefore the blob is actual.
This is patently ridiculous. I think the problem is due to an equivocation on the modal terms. People don't realise what they're saying when they call something 'possible' or 'necessary' (let alone 'possibly necessary'!) within the S5 system.
Let me use the term "quasi-necessary" to refer to any entity that is necessary if it exists at all; but without prejudice to the question of whether it actually does so exist.
It seems we can conceive of such quasi-necessary entities, be they Gods or blobs. But it would be a mistake to call them 'possible' on these grounds. To make a claim about possible necessity is not just to make a claim about some possible world, but rather the entire collection of all possible worlds. To say that Nec(X) is true at some possible world (i.e. "possibly true") just is to say that Nec(X) is true, simpliciter. (See here.)
Now, we can of course turn the argument around and point out that if it is possible that a quasi-necessary entity not exist, it actually must not exist at all. After all, to say it's possible that not-X, is just to say that X is not necessary. Since it is patently obvious that it's possible for God to not exist -- atheism is not inconsistent -- it follows (via the same logic as before) that God actually does not exist.
Theists will claim this argument is question-begging -- and I agree -- but it is no different from the earlier argument for God's existence. My point is that it is question begging to make any claims about the possibility of quasi-necessary entities. Claiming it's possible is just the same as claiming it is actual; and claiming it's possibly false is the same as saying that it's actually false. No progress can be made here.
Dinner Table Donts highlights what I think is the core confusion here:
I find it hard to think that God is not at least possible in our given context. Are the intuitions of millions of people, over millions of years suddenly just wrong about a Higher Power? I know the evolutionists will eat me alive for this, but doesn't the intricate design of our world and our universe at least allow for the possibility of a creator?
This confuses epistemic with metaphysical possibility. Given our evidence, it is possible that God exists, and also possible that he doesn't. Either is consistent with the evidence. (Of course, that isn't to say that both positions are equally justified.) But, as already noted, any quasi-necessary claim cannot be both (metaphysically) possibly true and possibly false, for that would make it both actually true and actually false, which is a contradiction.
Going back to my quasi-necessary blob, it seems possible to us that it might exist. At least, it is epistemically possible. (I certainly can't rule out the possibility of a necessary blob.) But, assuming that it does not actually exist, it follows that, due to its property of quasi-necessity, it isn't even (metaphysically) possible. The same goes for God, if understood as a quasi-necessary being. His actuality as a quasi-necessary being would rule out even possible non-existence, and similarly his actual non-existence would rule out his possible existence.
This can be overcome if the blob is not quasi-necessary. Your average blob is metaphysically possible, even if it isn't actual. The same would go for God if we stopped conceiving of him as quasi-necessary. A quasi-contingent God, though actually non-existent, could possibly exist.
But a quasi-necessary God (or blob)? Sorry, not a chance.