God is so quiet

[SteveII]

I have already conceded that there is a possible world where there is nothing except abstract objections--which includes the concept of existence. We can certainly conceive of a world where there are no concrete objects. If you don't think so, show the logic--that's the whole point of possible worlds semantics.

I think the problem many of us have with "abstract objects" is whether they are discovered or invented.
Is mathematics discovered or invented?

Is the concept of the perfect circle a discovery or an invention?
Is the concept of existence a discovery or an invention?

If they are discoveries, then I'd agree with you on there being such a world, or realm, of abstract concepts.
But if they are invented, then they are contingent upon rational minds - minds which, as far as we can tell, are contingent upon working brains, which are contingent upon a whole plethora of biological machinery, which is contingent on chemistry and, ultimately, physics.... the same physics that popped out of the big bang through... who knows?... quantum fluctuations?
So the best guess we have for the only necessary thing is the framework upon which all of existence plays out: space-time.

I agree with your analysis about abstract objects. I for one do not believe they are real objects because I think all those things can be grounded in the mind of God. However, atheists have to wrestle with the question and the consequences of either decision. 

However, why do you say our space-time contains within it an explanation of its existence (the definition of necessary)? Most philosophers consider all concrete objects as contingent.

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‘Nothing’ cannot be even a possibly actual alternative to existence, because if were actual; if that possibility were to become a reality, it wouldn’t be nothing, it would be a real and actual some kind of thing.  I’m running out of ways to explain this.

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I have already conceded that there is a possible world where there is nothing except abstract objections--which includes the concept of existence. 

So, you agree that existence exists necessarily then?  So...any god would be superfluous, yes?

I agree that it does not make much sense to argue "existence" point because it does not create a problem for me or solve the problem for you. There still are possible worlds where there are not universes or universes of different kinds. Possible worlds that only have minds, You don't need concrete objects to have a possible world. Therefore our actual universe does not contain within itself an explanation of its existence--it could have failed to exist. If it could have failed to exist, it is contingent. If it is contingent on something else, what is it? It must be something that itself is not contingent (at least somewhere in the chain going back). There must be a first cause.