firstly, attributing to God a nature assumes his existence.
Oops. I meant the nature the God argument.
secondly, since the existence of God is assumptive, as you have stated explicitly, and since one's worldview is entirely shaped by acceptance/rejection of this assumption, it is axiomatic.
No it is not.
Let's use some analogies.
Axiom: 1 + 1 = 2
This is an assumption, and it is generally held to be true, and it is necessary as part of any known mathematical system. Hence it is an axiom, though it cannot be proved.
Assume: Square root of two is rational
This is an assumption, but it is not generally held to be true, it is not necessary as the alternative exists that root 2 is irrational. Hence it is not an axiom. (in this case, the consistency of the resulting argument has been disproved. In the case of God, it is incomplete.)
Maybe we are just arguing in semantics here, but...
he speaks only in half-truths and vague generalizations and simply regurgitates everything that is in pseudo-philosophical vogue.
It has been informed that he is quite cuddly and happy at times. But such a state is understandably very hard to observe...

If he bothers you too much, ignore him.
Let me just sum up my position:
I believe that significant in the universe exists only relative to an observer.
I believe that chance can account for complex form, and the mind is itself a manifestation of complexity - a holistic existence, rather than a fundamental one. Mankind can therefore be just a branch of that "chance".
I believe that an universe without god is as wholly consistent and logical as an universe without.
I believe that it is impossible to determine absolutely which of the alternatives exists.
Therefore, while acknowledging the possibility of any god, I choose to act on the assumption that god does not exist, because I believe it to be the better way.
Simple as that.