culturally-Christian atheists, mostly, followed by a lot of the less-mainline versions of Christianity
Your statement of "mostly" is an extreme (10x) overstatement.
The text above was from memory. I just looked it up and I was pretty accurate: https://www.pewresearch.org/religious-landscape-study/
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.