TL;DR: One's reality is exactly that. "God", if He exists, is inherently beyond one's reality (for me and presumably everyone else on this planet). Otherwise "God" would be another being or a phenomenon or something that we could maybe percieve and even comprehend at some point, but that's not possible. Even the Standard Model is more real than "God" is. Whether or not "God" exists, we can't know as we are.

I'll bite for the first question, since you so kindly mentioned epistemology. Whether or not there is a God, I can't know. I would say "we" can't know, but I shouldn't presume that "we" are all at most around the same level of reality that I am. The concept of "God" inherently implies that a "non-God" (such as me) can't find out the truth about whether there is a "God", and if so, the surrounding truths and lies. If we (back to normal "we") never had spaceships, telescopes, or anything else of the sort (indulge me), how would we ever find out the truth about the structure Milky Way galaxy? It would just be beyond us, but only technologically.

This kind of epistemological investigation is in some ways utterly antithetical to mathematics. In maths, one can set axioms and prove a great many things (not necessarily all things, though). It's almost like creating things out of thin air. However, we can't even know "God" unless we can become "God", or else achieve a similar level of reality.

We can set axioms to describe things within our own level of reality, but if there are axioms of some higher level of reality, we can't reach them like we are. If there is a higher level of reality with "God", then "God" also may be contained within a higher level of reality. We simply can't know. There is nothing "beyond" to know. The entire study of how we come to know things, epistemology, is thus bounded by this principle. It's not even an axiom, which is something taken to be true and then used to reason about things. There is no denying it, because what you perceive as reality is proof that you exist in your reality. You maybe don't even need a concept of "I" at this level, because your reality can't be anything but your reality.

Your sense of reality may be constantly changing, but there is some reality you are within at a given point in time. All your beliefs, your memories, your experiences, what you think are your memories and experiences: anything and everything that you "know" is your reality. Your reality is that which can't be dispelled as an illusion. An illusion is a concept that you reject as reality. Babies hardly seem to know anything, but they generally have shifts in mindset, such as suddenly understanding object permanence. Suddenly the baby's reality is expanded. Something that wasn't even considered as a thought suddenly becomes routine. Of course, things aren't always so drastic. As more cognizant (than babies, certainly) beings conversing on Hacker News, one may have suspicions, conjectures, doubts, and whatnot as to one's sense of reality. All that is part of one's reality, really. There can be no reality one "knows" aside from the reality one already "knows". Perhaps one's reality at time 0 becomes illusion at time 1, superseded by a reality that may be very similar or noticeably different. To tie in a bit of psychological mischief, perhaps one's reality is altered in dreams; still, if there is any thought of anything, it exists within the context of one's reality.

Haha. That was a lot. I'm 100% not trolling. All that was my sincere thoughts. As usual, I figured some of it up on the fly. I've never followed any philosophers or anything, and hardly even know of philosophical works. I think I'm sane. Oh well. You did ask for it.