I am not refuting God's existence, I am only making the argument that one cannot claim to know of God's existence with such absolute certainty. (One can only claim to know of one's own existence with absolute certainty).
"Nothing is certain, not even that" (Arcesilaus)
Most people would probably agree that the central epistemological problem is determining which postulates to accept as fundamental. That 'God exists' may be true, but it is possible that it may not be true. Therefore it should be rejected as any kind of foundational postulate.
I disagree that certainty or even probable certainty (whatever that means) is the criteria by which we decide fundamental postulates by which to reason. You can always ask 'why' to a probable certain truth, and the answer that usually comes back is that if it wasn't certain, then our ability to make sense of the world is all messed up. Of course, relativity and quantum physics did that a long time ago in their own way, and we simply got used to thinking differently. So, I would exchange the idea of talk of certainty (possible talk) with the idea that we base our concepts based on what is meaningful to our existence. Since God is very meaningful to our existence, there is a strong basis for accepting this belief as a postulate that we should believe.
However, that 'I exist' is unequivocal and must be true. The fact that my existence is self evident gives it logical primacy.
(Cogito ergo sum)
The term 'I exist', like Carl mentioned, is meaningless unless it is referenced and phrased within the broader context that we cannot make sense of the concept of 'not existing' and still contemplating an issue such as this.
So whereas you claim that God is axiomatic, I make the claim that existence is axiomatic.
(To be more precise, I would claim that in an epistemological context consciousness is axiomatic, but in a metaphysical context existence is axiomatic).
God existing is axiomatic in the sense that it is a conclusion that we have to draw in order to make sense of our existence. It is not absolutely necessary that we make that conclusion (as obviously there are atheists who live quite fine without drawing this conclusion). The difference is that there are many well-drawn and meaningful conclusions that people fail to draw, but they are all the worse off for failing to do so. For example, it is probably true that many good people refuse to think of airline travel as basically safe. Their unwillingness to accept that airline travel is basically safe might gain them a small probability of safety if they choose not to travel on airplanes, but the overwhelming disadvantage of refusing to fly because of this belief (or lack thereof) is detrimental to their ability to visit family, go on trips, take on certain jobs, etc, all of which could make their life experiences more meaningful.
That God exists is a belief, and it remains so regardless of the strength of one's conviction that it is true. Our belief in God's existence occurs only by virtue of the logical primacy of our own existence, and the consciousness implicit in that existence.
Well, everything is a belief, even that you exist. In truth, you might not exist - even to your own way of thinking. You might be a butterfly dreaming that he were a man named Kyle, whose dream is a re-occuring one. Is the butterfly you? It depends on what you mean by 'you'. You are not a butterfly (at least I don't think so), so the butterfly that imagines your existence is not 'you' having the dream, you are some made-up phenomena. Of course, that's preposterous. It's preposterous not because it is not possible, it's preposterous because the possible concept just doesn't mesh with our experiences which are thought to somehow line up with what is real. Since this 'possibility' doesn't line up with what seems like reality to us is like, we discount it even to the point to where we even discount it as a possibility - even though I see no reason why it is not at least imaginably possible. Hence, rather than label something as a belief as if that's a dirty word meaning 'to not take too serious', we have to label a belief as anything that humans (even one) believes to be true or whom a human could believe to be true.
What we want to know is, which beliefs are those which we can feel a certain amount of comfort in believing in that they make our world coherent and comfortable (pragmatic) by which to exist. To fully do so, we need a belief in God and to treat that belief as true. This is the best we can hope from a belief.