To a journalist who asked to Einstein if he believes in God, he answered : "Tell me what you mean exactly by God and I will tell you if God exists.". Often we argue about what we believe to be fundamental questions but are only words problems. That is why it is important to precise the meaning of the words we use, which is not always so evident that we think.
Let us suppose the following definitions :
From these definitions we may deduce :
If God exists, then it is part of the universe, because the universe is the set of everything that exists. Before the creation, the universe did not exist, then God, being a part of it, did not exist, which is a contradiction.
This does not demonstrate that God does not exist but that some too simple conception of God is contradictory.
One could believe that this contradiction could be avoided if we consider that the God creator of our universe is not part of it; the universe would be the set of everything that exists, without God. But the same problem would then exist at the upper level if we consider the super-universe made of God and our universe. Who created this super-universe ? Who created God ? A super-God ? And we could go on so indefinitely.
And even if we consider the universe made of our universe and all super-universes, the same problem would exist. We should imagine a God of "level infinite plus one", and so on... We would have a stack of universes and gods indexed by transfinite ordinals. Our god would appear rather small in comparison with the God of level epsilon zero...
Another more plausible conception would be the one of a supreme being which would have creating the universe and himself simultaneously. We could consider him as the spirit of the universe, the set of all spirit contained in the universe; the spirit of each living being would be a part of it.
Some physicists would like to fing God in physics, and think to have found Him with the anthropic principle. And they reject the many worlds hypothesis because it permit to avoid the notion of creating God, "Great Architect" of the universe, chosing the exact laws permitting the apparition of complex life. But the many worlds hypothesis is in fact the simplest to explain the anthropic principle, and the creating God does not appear as an anthropomorphic god chosing the laws of the universe, but as an abstract metaphysical principle, the fundamental metaphysical principle.
Independently from the "Great Architect", we may also consider God as the spirit of the universe seen as a living being, or a living being who created a sub-universe, or a superior being who leads us.