[Serious] Could an omnipotent and omniscient god prove that he was God?

[Neo-Scholastic]

Abstractions are based on facts.

No. Abstractions are based on observations. Careful observations result in more reliable abstractions than sloppy observations or abstractions based on suppositions about how things should be, such as Manifest Destiny or Sin. Sin is an abstraction. From your perspective, on what fact is it based?

They [abstractions] are not just some made up thing that bears no resemblance to reality

I agree. As stated above, careful observations produce better models of reality. Those models of reality (abstractions) should never be confused with actuality. Natural science makes better observations than did pre-modern folk physics based largely on suppositions. The finger pointing at the moon is not the moon. The world-as-we-understand–it is IMHO a vast gloss over the world-as-it-actually-is.

...the axiom of existence is not the same as saying there's a god which is necessary for everything to exist.  No, it says everything already exists and there's no need for a god to create it.

And what is everything? The universe? ….just by what…definition. What if I say to you that Everything (in classical philosophy “the All”) is more than the empirically knowable universe? IMO some parts of reality can only be accessed by reason. Change is constant. Speaking from the perspective of presentism, it seems that the universe over-writes itself. The universe of each version of now is a different universe from the one “before”. And it makes one wonder. Things come into being and things pass out of being, yet some things always remain the same. They are eternal. The debate starts with Parcelsius and Heraclitus and works its way into the First three demonstrations of God’s existence by Thomas Aquinas which remain logically sound.

A very common mistake of atheists is this. They mistakenly assume that God is separate and distinct from the physical. And yet it has been Christian doctrine for hundreds of years that God is simultaneously immanent in and transcendent over creation, i.e. the physical world. To me that corresponds neatly with two basic metaphysical concepts: 1) what is eternal in every transient moment and 2) what must be true in all possible worlds. To me, “metaphysical” is not a category of extra stuff over and above physical reality but rather the necessary and sufficient reasons or causes for there to be any given possible physical reality. This logical chain of contingency traces back to a necessary being, the All, that includes the physical within it but is not identical to just the physical. This perspective is called panentheism.

You say that the law of identity and the law of non-contradiction are absolutes and they are even though they are abstract principles because the facts on which these two abstractions are based on are absolutes.  Every fact is absolute and necessary.  To exist is to be necessary.

Whoa there…just to be clear I see a very important distinction between abstractions, which are passive descriptions of reality, and the active causes so abstractly represented and/or modeled. Descriptions have no power over reality. The idea that they could is called magic. And I doubt either of us believe in magic. (quiet, Nudger) IMHO actual reality is most likely very different from our best suppositions about it. And no, some facts apparently need not be as they are...the physical constants do not seem absolute. Nothing seems to exclude a possible world in which the speed of light was just teeny tiny faster or slower or the gravitational constant a little higher or a little lower. And no, to exist is not be necessary. Mosquitoes exist and I can so no reason why they are necessary.

Your formulation, that some supernatural being is necessary for existence to exist treats existence as a derivative or contingent fact instead of an absolute which all facts are contingent on…If you are pointing to something that exists as an explanation for existence then you haven't explained existence.

No. I am saying that the problem of universals cannot be skated past so quickly. I say, the universe is greater than the sum of its parts, particular beings that come and go. Things within the universe are constantly coming into and out of existence. The composition of the universe is constantly changing. So what happens when all particular things cease to exist in a future possible now? Why doesn’t it just all cease to exist?