Response #1/ No bad experience. I realized that there is no proof that god exists, in any form, and thus cannot devote my life to one.
Response #2/ [This bad incident] was so overpoweringly awful, that I realized no existing god would allow it.
This statement can be both accurate and exceedingly offensive.
Accurate if there was, in fact, a specific instance that made you realize that god did not exist. Offensive if your atheism came about through years of struggle, with both your family and friends, insane amounts of soul-searching and reading and thinking things through in a logical manner. Because it undermines everything you went through.
The struggle from religion to atheism is generally an uphill battle, met with nothing but resistance and threats of eternal punishment. For that reason, many deconverters make the journey alone. And to suggest that it was a bad sip of wine or a nasty look from your priest takes away all that you’ve accomplished.
There’s a saying in the atheist community that a Christian becomes an atheist once he or she has read the Bible and, further, that the Bible is the atheist’s best evidence to show that religion is wrong and god doesn’t exist.
There’s fact in these statements. Not only is the Bible full of contradictions, but the Christian god is not an all-loving, all-forgiving god, either. As higher-functioning animals, humans are capable of logically developing a scientific process that eliminates what seems trivial or untrue. Logically, the idea that a god exists at all is irrational because there is not a shred of evidence (aside from the Bible) that says otherwise.
Also, the religious person who says this can also be suggesting that a lack of personal convictions. I think it would be safe to assume that it would take more than a single, small incident for someone to completely give up their faith in god. Ask the religious person what it would take for them to give up god. I’ve generally found the answer is either “nothing” or “strong proof to show the Bible was fabricated.” Then explain how you feel you’ve found evidence that the Bible is fabricated and why the stories don’t make sense. That you logic’ed it out.
Lastly, if you’re getting into a debate about religion v. non-religion, how you came about being an atheist is completely irrelevant. You could have decided to become atheist from choosing the option from a hat. That’s not the issue. The issue is whether you can form coherent arguments against religion.