It's Christianity by a wide margin (about 2/3rds of the population). I'm not sure where you get the idea there are more masonic lodges than churches. Just a quick check for my state, Massachusetts, shows about 300 masonic lodges and about 4000 churches.
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.
One Masonic Lodge vs 18 churches. Repeat that experiment with a bunch of random cities and you'll find that outside of the very high population cities (NYC, Chicago, Atlanta, etc.) there are rarely more than one or two lodges, if they even have one at all.
> I noticed that there you have more masonic temples than churches in every city.
... Eh?
The US probably has more churches, as in physical buildings, per capita than anywhere else in the world.
Freemasonry isn't a religion, though it has vague religious trappings, but in any case it's fairly absurd to suggest that there are more masonic buildings than churches in the US.
I noticed that there you have more masonic temples than churches in every city.