The existence of God is not a necessary truth but a contingent feature of the way reality is that one can only discover a posteriori. Theism is an interpretation of reality according to which God's existence is necessary, using arguments such as the Modal Ontological Argument (MOA) to establish the existence of a necessary being. The argument goes the following way: P1: There is an x such that if x is possible, then x is necessary. P2: x is possible. C: x is necessary. What the argument fails to establish is that there is any particular concrete being that can actually satisfy the definition for x. For all we know, the only thing which is capable of satisfying the definition for x is the empty predicate. And there may be a further problem: The Theist is committed to a non-univocal conception of existence when they predicate existence of God, since existences are contingent for all concrete entities. As such, the atheist is not saying anything false when they say that God doesn't exist, because their conception of existence is contingent. So, for the Theist to say that the Atheist is making a false claim when the Atheist says that God does not exist, they have to show that "existence" includes different modes of existence, and that it is not univocal across all entities, but then they are just using a different notion of existence when they claim that God exists.

A necessary being isn't the same thing as God as a matter of fiat. One not only needs to show that a necessary being exists and grounds all the contingent facts, but that the being in question cannot fail to exist. The problem is that the Theist typically understands necessity under a possible-worlds semantics. They say that if God exists in one possible world, then he exists in all possible worlds. But this is only true of all the worlds in which the necessary being necessarily exists, and the Atheist can just say that the Theist is ignoring all the possible worlds wherein God does not exist. Theism also seems to force you to adopt a more strained and restricted view of modality in order to make "God is necessary" come through as true, adopting certain metaphysical assumptions which could be coherently doubted. And it just seems obvious to me that worlds where the proposition that "God does not exist" is true are fully conceivable. I've never even claimed that science can give a final answer, as this assumes that I'm claiming that there is such a "final answer" to speak of. My claim is that nothing that exists necessarily exists without importing a non-univocal definition of existence which is dubious from the beginning, because I take it that existence is univocal and that to speak of different modes of existence (such as contingent and necessary existence) is fundamentally problematic. How can the Theist show that God is necessary without begging the question?

There's still a glaring issue with your argument, which is that you don't actually show how "God" in particular gives you the grounding that you're looking for. And I think that the important kind of existence is the kind that we use in talking about concrete objects, so that when an atheist says that God does not exist, he is saying that "God" (as far as we know) is not a concrete object, which the Theist wouldn't be able to object to. The fundamental "ground" might be nothing more than an abstraction, an artifact of certain discursive tendencies, rather than something that actually exists in a concrete sense. Heidegger says that "The being of beings is not itself a being" and I think he's fundamentally right about this, insofar as being is always being in a certain way, so that any conception of God that asserts that God is "pure being" becomes meaningless or un-analyzable. Pure being is an empty qualifier, an empty predicate, such that to say that something is pure being is indistinguishable from saying of it that it is pure nothing. What is wrong with saying that the universe is self-sufficient? Is it any less problematic than saying that God is self-sufficient? Anyway, we've lost track of the thread, because the original debate was about God being a necessary being, and I contend that nothing that exists is necessary, and that we can coherently conceptualize worlds wherein God is absent in complete detail, so there is nothing analytically necessary unless one wants to make further claims which themselves can be coherently denied, since such claims are not true as a matter of a priori necessity, and they only restrict modality. I would argue that a looser modality, a more liberal modality, is far preferable for talking about the full range of metaphysical possibilities than a modality that forces "God exists necessarily" to be true across all possible worlds, which just makes it so that worlds which seem coherently possible must therefore be impossible. But that's just arbitrarily stipulating that such worlds cannot exist because "God is necessary" can be made to seem true using certain modal logic axioms which are themselves treated as necessary when they are merely stipulated.

Well, the debate is about whether "God" is necessary, rather than any old metaphysical ground. Now, why must the ground be a being, or the greatest conceivable being, even? Why must the ground exist "in all possible worlds"? To be able to conceive of something and to be able to deny it are two different things, so that there is never a contradiction in denying anything which can be conceived. Hume says that whatever can be imagined to be, can be imagined not to be. This I think is a compelling reason to reject the notion that there must be a necessary being, since we know of being in terms of that which fills up the empty gaps in reality. Without those beings, there are gaps. But when you strip away all beings, what are you left with, if not a giant gap? And I don't mean a logical gap (there is no contradiction in admitting absence), but an ontological one. When I imagine that something is "there", I can always imagine it being "not there", of the absence of said something. This will just be true of all things which can be imagined, so that if God exists (and his mode of existence is unlike anything we have ever seen, in that he cannot even be thought of as non-existent) then he is unimaginable and inconceivable, and if this is true, then I have no idea what it means to even talk about such a being, or to say that such a being "exists" in this non-univocal sense. One could simply chalk it up to philosophers "making things up", inventing ideas to make up for the gap in their understanding of reality in their attempts to make sense of the strange world in which they find themselves, grasping at intangible abstractions whose real meaning and implications cannot be coherently understood, even if they (the philosophers) pretend otherwise. Rather than inventing new concepts, we should prefer to use science and reason to discern whether our basic assumptions hold real ontological weight when applied to reality. Does the world need to make sense to the philosopher? That is a moral prescription, and reality is not bound by our dogmatic insistence that it obey "rational" thought. The world seems to be a miracle in itself, but does the miracle require a deeper miracle to explain it? The world subsists on its own without the need for an ultimate foundation, because talk of an "ultimate foundation" must be grounded in empirical reality, and not in metaphysical speculations from the armchair. Is it any more reasonable to insist that reality must depend on a necessary being (which has already been shown to be a dubious notion in itself) than that the world just exists? If an ultimate explanation were needed, would it not be hubristic to think that we had access to that explanation by way of pure a priori reasoning? And what if modal logic itself requires a deeper grounding? What if modality is itself contingent and context-dependent, so that necessities of the sort that the MOA utilizes are only locally necessary, as opposed to necessary across the whole range of possible worlds? All of these arguments make it look like the "necessary" God is not so necessary after all.

If the answer to why there is anything at all is that there is a being-who-is-also-not-a-being that both exists and yet is empty of all properties that "grounds" all the contingent features of reality, then you have offered a non-explanation. God might as well be no different than The Great Om. The unspeakable, ineffable, and unintelligible "ground of being" who is yet nothing at all. An empty explanation is indistinguishable from no explanation at all. And if we cannot inspect God, or even the concept of God, then how is there even a thing being referred to? I'm not saying that the universe grounds itself (though at least we can actually see the universe and count it among the existing things), but I'm also not completely opposed to the idea. And so, in lieu of all these considerations, the necessity of God seems to have been completely nixed, deflated of all its explanatory power. I think that none of these arguments have been seriously addressed, and that the Theistic account has been made more implausible and impotent as this debate has gone on. I just don't see how the emptiness of the God concept counts in favor of the Theist, because it basically sounds like an admission that the Theistic explanation can no longer be called an explanation at all. What is it that the non-thing that we call "God" explains, or could explain, at all? I might as well just counter that the Atheist has another account, The Dark Concept, and that it explains everything (even if it means nothing). How is the Theistic explanation any different from The Dark Concept or The Great Om? No real symmetry breaker has been given thus far. But what is asked is not merely for such a symmetry breaker to be provided, but that the Theist can actually defend the idea that a necessary being God exists, or is even coherently conceivable, without begging the question in favor of his version of modality. I would simply retort that a looser conception of modality allows for a greater range of possibilities and that he will have to argue for a form of modality which is more stringent and constrained in order to make impossible the worlds where "God does not exist" comes out as true. My opponent says that God is the precondition for any inventory, but he hasn't described anything that can do any kind of explanatory (or grounding) work, since his version of God is still indistinguishable from The Dark Concept or The Great Om. And even if he can coherently describe the concept, there are also the issues that I have with a non-univocal conception of existence and the fact that denying is separate from conceiving (so that one can always deny what they are able to conceive without contradiction). Given the full range and force of my objections with the concept of a necessary being, my opponent will need to climb a tall mountain in order to rescue his necessary being from annihilation.