The Gods do not exist “in” possible worlds, but rather cause them to be. Their necessity is thus not captured by contemporary modal semantics – which concern beings, and not those who are beyond being. To illustrate, consider that something could exist in every possible world, but only
because it is caused to do so. The “existence” of the Gods is better captured by something like “aseity” than “necessity.”
Be that as it may, I’ve got a number of
a priori arguments for the Gods. Here’s one I used to play around with, probably more relevant than others because most of the folks here are Catholic:
D1. Epistemic potentiality =df. the potentiality of something,
as far as we know.
D2. Epistemic actuality =df. the actuality of something,
as far as we know.
D3. Epistemic possibility =df. the possibility of something,
as far as we know.
- Whatever is epistemically possible is either epistemically potential, or epistemically actual.
- The existence of a pure actuality is epistemically possible.
- Therefore, the existence of a pure actuality is either epistemically potential, or epistemically actual. (1, 2 M.P.)
- But, the existence of a pure actuality is not epistemically potential.
- Therefore, the existence of a pure actuality is epistemically actual. (3, 4 Disj)
(1) makes sense because if something is possible as far as we know, then, as far as we know, it’s either an actual thing, or a merely potential thing. Non-tertium datur. (2) is practically indisputable, and is the weakest possible modal premise. (4) is necessarily true, since it is a contradiction in terms to say
pure actuality is potential, as far as we know. (5) doesn’t get us “a pure actuality exists”, but it gets us “a pure actuality exists, as far as we know” which is functionally equivalent.
Now, full disclosure, I think the Scholastics got thangs awfully wrong, and so I wouldn’t talk about the Gods in terms of “actuality” in a more formal setting. But, getting an atheist to admit there’s a pure actuality is significant enough that I’d play along in order to move on to the
real stuff.