What if someone asks why you don’t believe in God? While it’s a somewhat rude question, and therefore somewhat abandoning any atmosphere of politeness, that’s not necessarily an excuse to be rude in your answer.
Yes, it’s rude to ask, because it’s a personal question, something people don’t generally talk about. Oversharing is a bit rude, and therefore so is inviting someone to overshare. But a person who asks why you don’t believe in God isn’t necessarily (or, indeed, probably) being deliberately rude, and they may be genuinely interested in deep discussion (the sort that leaves behind niceties like etiquette, not to be rude but to be serious).
So, in answering such a question, the issue is whether to answer in the manner of an intellectual debate or as a personal journey (what Christians call “testimony”). Either option allows you to be politely vague about your reasons/experiences, although talking about your experience is less likely to come across as confrontational.
But any way you slice it, explaining why you don’t believe what someone else believes, or how you used to believe it but don’t anymore, is likely to come across as some sort of criticism of that person’s belief.
If you explain yourself in a relatively non-confrontational way (e.g. “I just found religious answers less and less satisfying as I went along in life,” or “I was raised that way” or even “I’m still looking for evidence of some deity’s existence”), and your questioner still tries to take offense, you can politely explain that you’re only answering the question they themselves asked. If they didn’t want an explanation of why you found theism to be inadequate or illogical or just plain silly, they shouldn’t have raised the question in the first place.
BTW, my own best answer to the question is this: Either human psychology was created by an all-knowing, all-powerful, all-loving God, or said omni-max deity was created by human psychology. The more I learn about human psychology, the more the latter proposition makes sense.
(cartoon via Chris Madden)