There is probably nothing that we can claim to be true with 100% certainty if you have a sufficiently creative brain, so it comes down to a level of certainty based on available evidence. The only current evidence of the existence of the God of the Bible, or any other gods are writings by men. Men who, in that period of time and before, witnessed a world which they had no ability to scientifically explain. Men who killed and buried animals under buildings hoping to appease celestial beings and bring good luck. Their writings are the only current "evidence".
That being true, I don't see any reason to believe in gods.
All one has to do is apply the same requirements for belief in gods that you apply to anything else in life, to become a non-believer. You aren't going to read the Iliad and the Odyssey and start believing in those gods, right?