As an American, I (roughly correctly) estimated Christians to be about 65%, and my wife and I both estimated athiests to be about 20%... I guess "athiest" is too strong of a term here, and most people (self included) would more accurately desribe themselves as non-religious or unaffiliated.
Atheism is a dirty word in America. Way more people have no particular religious beliefs, but they’re turned off by the word.
I'm not even sure why atheism has a name. It doesn't feel like a category that anyone would identify with. Lots of people are simply not religious which is more like being null in the religion category while atheism is like -1.