Disbelief is not simply suspended belief, where you think something exists and yet deny it. Disbelief is the state of not believing.
I assume that by "agnostic" you mean "unsure of whether or not there is a God". This is a mistake people make in terminology. "Agnostic" is an epistemological term describing the belief that we cannot
know God. What you probably mean is that you're a
Deist, in that you believe that there is probably some kind of supernatural entity, but he doesn't perform miracles and he doesn't interfere in human life (though he may bestow divine revelation). This is the category that the Founding Fathers of the United States fell under,
not Christianity, contrary to what every religious political figure in America would have you believe.
When people ask me my religious association, and I reply "I'm an atheist", they commonly ask "so you don't believe in God?". To this I say "which God? Zeus? Thor? There are thousands to choose from, I just don't proscribe to any while you proscribe to one".
And inevitably they try to vindicate their decision with a number of flat-out subjective arguments, eventually coming to the Bible. At this point I promptly beat them over the head with a science textbook to drive home the point that there's nothing special about their faith, and then walk away.
So when you ask how can I not believe in something that I don't believe exists, well that's just needlessly complicating the point. I don't believe that
any god exists, and I think that anybody who picks and chooses one out of the multitude is being arrogant in thinking that their subjective perspective is correct.
So, for the next point of debate, what makes the God from the Bible seem like the most likely (but, yet, also not certain) candidate for being real to you?