The Bible provides evidence that can be tested to validate its reliability consistent with natural and human history. When we test the Bible’s witness, we find that the Bible is a reliable witness of the history it reports. This is true, by the way, not just of the Genesis flood narrative but for all the various historical accounts contained in scripture. Since the Bible is accurate according to the examination and application of external sources of evidence, we can have greater confidence that it is telling the truth in areas we can’t test.
God’s existence is a logical necessity. R. C. Sproul said that as long as you believe that anything exists– even just a pencil or an old shoe – that from that single fact, you can deduce that a self-existent God is necessary. The fact that anything exists today is a graphic demonstration that there has never been a time when there was absolutely nothing. The universe that we can see points to something larger than itself. Philosophers, even ones with no connection to Judaism, Christianity, or the Bible have made the same observation. Aristotle, for instance, reasoned that somewhere there must be an “unmoved” or “prime mover.” So when Christians assert that the universe contains evidence of God, or a Necessary Being or an Unmoved Mover or whatever, they are really not saying anything that hasn’t been recognized for thousands of years.