Dear Internet Diary,
Did you have a bad experience with your former church or religion that caused you to reject it and become an atheist?
This is a trick question. It's like asking, "When did you stop beating your wife?"
If one is an atheist, but was indoctrinated in a religion, there is little chance that something bad didn't happen. I mean, sure; whose Christian school-teacher didn't tell her that when God made people out of clay, He left some too long in the sun and "ruined" them, and that's where we get black people. The post-hoc assumption the theist desires to make is that the bad experience caused the atheism. The accusation in this is, "The atheist is only an atheist because he interpreted the bad actions of someone as Christianity itself. He/she is guilty of a logical fallacy."
Not that all atheists are logical.
What the Christian or Muslim or whoever is looking for is some kind of juicy gossip that might result in some love-bombing, Christian council, and maybe a good, down-on-your-knees, repentance session. The best possible answer is, "Why, yes! A Youth-Pastor/Priest molested me as a child." If that doesn't prove the truth of religion, I don't know what can! Luckily for the questioner, there are so many hundreds of thousands of these instances, their faith is safe. If a math teacher hurts you, does that make math untrue? If a baker yells at you, does that make cake any less delicious? If an astronaut clocks you, does that make the moon-landing fake?
Unfortunately for the questioner, there are other kinds of "bad experiences with religion." They're called the recognition of logical contradiction, moral and ethical problems, and "breaking the laws of nature." The Problem of Evil. The Argument from Non-Cognitivism. The most ethical priest, the most beautiful-sounding choir, and the tastiest church potluck Jell-O salads can't mask those bad experiences.