The first thing to consider is that "God" is an ambiguous term. What is meant by "God" differs literally from person to person, even among people who claim to follow the same god from the same religion and even are part of the same denomination. So, before you even begin your debate, you must first establish what is meant by "God." This should include a list of characteristics, any dogmas pertaining to that deity, and any historical actions that god supposedly participated in. From there, the debate will go one of two ways: 1) The defined God does not exist because its characteristics and/or actions are refuted by historical or scientific fact or logic (Example: The Young Earth Creationist's version of Yahweh from a historical and scientific point of view, or a god that is omnipotent from a logical point of view, since omnipotence is self-contradictory) or 2) There is no verifiable empirical evidence that such a god exists, therefore belief in that god is not rational. There is no third category; belief that any god exists is irrational and unjustified since no empirical evidence indicating that such beings exist has been shown. How do we know this? a few reasons: 1) Studying gods is not a science, but instead a matter of cultural studies. 2) No Nobel Prize has EVER been rewarded for discovering concrete evidence for the existence of any gods. 3) There is disagreement among every believer in all gods regarding that god's characteristics and attitudes towards things. Why else do you think, for example, that Christians cannot come to consensus on if the 6 day creation was literal or figurative, or if Hell is an eternal place of torment or if it is just "separation from God" or if it is nonexistence? If there were any actual evidence for the existence of any gods, none of these observations would be made. While the first option (where "God" actually violates what we know about the universe) is the only one where you can prove that God does not exist, the second option still lets you win the debate because no verifiable empirical evidence = no good reason to believe god does exist. This is the heart of the null hypothesis and examples of it such as Russel's Teapot or Carl Sagan's "Dragon in My Garage." By taking the side you have, you have already won the debate because the person on the side of "God does exist" has no case that doesn't rely on personal beliefs or appeals to unjustified authority.