I seriously doubt that etiquette is particularly universal in the US anymore, Everyone kind of has to examine their feelings, and maybe get some general opinions like you are doing here, do our best to not be rude, come to accept that different groups of people have different sub-cultures, and if in doubt, ask yourself if it is important enough to you to find out if a person belongs in a closer circle of your friends to risk whatever level of alienation might happen if they are offended.

I tend to just not even think about religion unless someone else brings it up, so it almost never occurs to me to ask unless someone else brings up the topic.

Whether or not I mind being asked totally depends on how judgmental I think the other person will be about my answer. I'll answer though, unless the interaction has already gotten to the point that I don't want to converse with this person anymore anyway.

I have not read Miss Manners on this but I usually don't bring it up because it up.As long as someone is not trying to convince me it is not a matter of conversation. If I were considering marriage that would be a different issue

Depends on the context, but I say yes - go for it. Because emotions can run deep when talking beliefs and values, tread lightly and be respectful.

If I feel they won’t freak out.. I’ll put out that ‘I’m not religious’ … in a non aggressive tone, and only hope they’ll respond in kind.. It’s amazed me how many new friends turn out to think very much as I do, which shouldn’t be that much of a surprize given we’d for some reason been attracted to each other.

Sadly, the opposite can happen. Usually it’s ‘them’ injecting religious elements into their statements, likely to ‘test me’... Whereas I do my best not to show deep disappointment, forcing me to limit our friendship before it deepens. I’ve learned many times over, you can only go so far and so long with a religious friend before hitting the wall, damnit.

I wouldn't ask normally for its something that I don't find relevant very often.

Heck yes... go for it!

I don't mind people asking me. I'll gladly and proudly tell them I'm an atheist.

it's a harmless question or should be. religious people knock on your door to do it.

But seriously, I'm surprised it hasn't come up. Just bring up current issues in politics, like Roy Moore, or the Supreme Court recently hearing arguments about that baker refusing to serve that gay couple. With so many Republicans trying to inject religion into politics, a lot of issues touch on the subject.

You could say "So, have you been saved... from religion?"