Because God is the One who brought everything into existence. We wouldn't draw even one breath without God allowing it. Without the God of the Bible, we would have no foundation to even reason how someone can exist if they were not created. Therefore, by the very fact that we can reason anything (arguing for or even against God), the God of the Bible must exist. It's a bit more complicated. I can find a couple articles about Reason and Logic and God, if you'd like.
...and I FOUND THAT THREAD! It does exist after all...
http://christianchat.com/bible-discussion-forum/133112-who-made-god.html