That's an interesting question because the religious love to ask of the non-religious, what they would do if they discovered god existed. Yet many refuse outright to entertain the hypothetical in the opposite direction. As an atheist, discovering that god doesn't exist wouldn't be all that surprising to me. Unfortunately, even if there was definitive proof that he didn't exist, people would still believe regardless. God represents an easy road to morality, decision making, and the promise of eternal happiness. So, while I think this is a great question, I will be legitimately surprised if you find any theist willing to answer it.