In my opinion, it leads to the fact that we cannot determine God (or lack thereof) using pure reason or anything else.
Absolutely right. This is the sticking point in the God Battle, I think. I have said this before, but my opponent insists that Reason leads to proof of his existence (precisely because the universe itself doesn't show God - though she didn't answer my question: "If God created the science we now discover, why did he make it so astoundingly difficult to find proof of Him in that science?")
Even if there must be a "first cause" (and it's not clear that there must) there is no evidence by which one would assume it to be God. "We don't know what caused the universe" isn't the same as "God caused the universe."
My issue with the "First Cause" argument is: where do you decide that the thing which caused our universe is the First Cause? Why not the thing that caused the thing which caused our universe? Or the thing which caused the thing which caused our universe? And once you have determined that, which one of that string of creators is God? Is God that which created us, or that which created the creator of the creator?
Also, in reading the God Delusion (which my opponent is doing now), she seems to be taking the opinion that Dawkins is trying to say "God is unprovable," not that God is unobserved to the "there is zero evidence that he exists." And though I have reiterated that, it seems to not being understood. Why is the big misunderstanding by most people the concept that Atheism is a "faith" with all the answers of denying God, when Atheism is quite obviously the state of waiting for the answers? I'm sure there are those who believe without doubt that God doesn't exist, but everyone on this forum, and all the main atheists in the world don't deny him, merely say "We don't see any reasonable evidence that he is there and the universe gets along fine without him."
I just don't think that God can be proved or disproved, so although it's certainly an interesting discussion, go into it realizing that there have been no definitive answers in the last several thousands of years and you won't likely stumble upon one during the course of the discussion.
Have you noticed that most arguments tend to use answers from thousands of years ago, though? I'm sure there are plenty of atheistic examples, but I notice that the names which come up frequently are Plato, Aristotle, the Bible, and more recently Descartes, Anselm, etc. Is it telilng that "pure reason" which proves God comes from before an extended knowledge of the universe? Shouldn't reason incorporate the changing knowledge?
I think I understand your argument, but I think that logically God must exist outside of the universe in order to have created it. It seems that "somewhere" as you describe it must be inside the universe.
Maybe it is my limited 3-dimensional mindset, but assuming God exists outside the universe, then creating the universe created that universe "somewhere." I'd say that that could be argued for God-less existence, too, but at least making the assumption that God existed to create the universe, then there was something (God) existing somewhere to create a new something (the universe) where the first something (God) was. Even if the universe was a subset of God, as you say, there had to be God somewhere for the universe to be a subset of.
I'm not certain I understand what you mean by "there must a means to measure or define existence by virtue of knowing that it exists."
Sorry... I'll try to clarify:
Also, just to point out that from my actual reasoning earlier, I'm assuming that God exists to show that the contradiction that he can't. I take the definition from Anselm's argument, and make a response to it.
In our universe, Gravity, Electromagnetism, and the nuclear forces define its existence. Without those forces, matter would not have any coherence at all. I don't know quantum mechanics well enough to say that if matter had no coherence, it doesn't exist entirely, but our universe is defined, and we can exist in it because of those forces, and the three dimensions (plus time). I wonder if the Big Bang would have happened without all of those things operating in some form on the matter in the singularity (but I'm a history student, not a physicist

)
Extending that, if God exists "somewhere," then there must be something which defines how that somewhere exists or operates, otherwise nothing could exist there. It doesn't have to be our fundamental laws or dimensions, but to have space for a thing such as God to exist in requires some establishment that it is there in the first place. Does that make more sense, of have I even confused myself?
In fact, we don't know that God exists (even if he does), therefore I'm not certain that there must be a way of measuring or defining his existence (even if he does exist).
Just to refer to above, for the purpose of my original reasoning, I assumed God existed. Otherwise, hell if I know if he's there
In my opinion, you seem to be approaching it with the correct attitude! Good luck!
-Bri
Thank you, and after lurking for like four months, I knew that if I could get constructive discussion on arguments, this would be the place to go

I hope I haven't hijacked the original thread too much.