Pew polls show about 5% or slightly less identify as atheists. About 30% (including atheists) are non-religious. About 60% identify as Christian. Then a few other groups that are 1-2%: Jewish, Mormon, Buddhist, Muslim.
Your statement of "mostly" is an extreme (10x) overstatement.
I take umbrage at another aspect of your statement: "culturally Christian atheists". I've heard this pointed specifically at me by people saying things like, "You are Christian, you just don't know it." That say that because I tend to be sensitive/kind/helpful/low ego. Yes, good Christians should be those things, but that doesn't mean that Christianity owns those traits or that those traits didn't exist and weren't valued before Christianity came along.
If you asked the typical American protestant (who does NOT go to church every week BTW) if they really believe in the whole god and heaven and hell thing, a lot of 'em are gonna hem and haw. It's closer to a social club than an old school religion.
I don’t disagree! but among Americans who do not go to church, most of them have parents or grandparents or great-grandparents who attended Christian church regularly, and some of that culture is still prevalent in subtle things like “protestant work ethic”
Evangelical Protestants, Catholics, and mainline Protestants make up 60% of the US population. Atheism isn't that common, maybe 5-10% of the population depending on definition.
