I believe you are still combining the ideas of "infinite" and "all-encompassing" together. While some types of faiths would hold that God is all-encompassing (and actually there is not a problem here either), the majority simply require that God is infinite, but not necessarily all-encompassing. For example a line is infinite. It is not bounded in either the positive or negative direction. However a line is properly contained within a plane. The line is infinite but not all-encompassing with regard to the plane..)
Note: The definition of infinite is endless or unlimited. A line is only partially infinite as it stretches endlessly only into one direction and not in all. Hence, it is finite in all directions but one.
The crux of Mycroft's argument is this: Most Judeo-Christian faiths describe God as distinct from the universe. This is incoherent because the universe is traditionally taken to be all that exists. God is maintained to exist and to be distinct from the universe. Hence, simultaneously two incompatible claims are made. God exists and God is nothing (outside of all things that exist). You have not yet furnished a refutation of this view.
On that note, we may drop the argument about the infinity of the universe. The main question is about the separability of God from the universe. If he is infinite (has no limits), then how is it possible for his nature to not include the world? Would you reply with a claim that he is infinite but not all encompassing? Exactly how a line is infinite, that is God stretches infinitely in only one direction and therefore does not occupy all of the universe? In that case, he goes through the universe, but then where does he go after he has gone through his creation? Into nothingness, or ceases to exist? In that case he is not infinite like a line, he is a segment. Or not infinite at all. If he is infinite like a line (partially infinite), then he must proceed infinitely into the endless (infinite) universe that he created. Perhaps, exactly like a line he occupies only a part of the infinite plain. But in that regard, his nature must be coterminous with at least part of the universe and not completely distinct from it as many Judeo-Christian theologies maintain. At best, they would have to concede that God is only partially infinite, rather than completely.
Supplemental Note: Theologians may respond that it is not the case that God exists outside of the universe. He exists within the universe but outside of
our universe. He resides in a realm that is radically different from ours. That is, one outside of time, space and matter. In that case we cannot talk about his existence meaningfully. When we claim that something exists, we assume that it has some characteristics relevant to our sensual perceptions. Such as for example, occupying space (being finite or infinite, that is occupying some space or all). If we say that something exists outside of space (God), we claim it occupies no space, or it is nothing by definition.
On that note, the talk of God existing in a different universe must be dismissed as hopeless non-sense. It means the same thing as God existing outside of the universe or in nothingness.
(Well you're whole post is very well written, so I guess it's better that you did.) Yeah the OP confuses the ideas of "infinite" and "all-encompassing". The initial premise is flawed, so the whole argument is pointless.
