How would you even define "Christian" these days?
Most Americans claim to be Christian, but when they have cancer, do they pray to God, or go to the hospital?
I dare say 90%+ of "Christians" don't actually believe God/prayer will cure their cancer
Does that make 90% of them atheist?
Also note that in America, "Christianity" comes in many flavours... Protestant, Catholic, Mormon, Jehovah's witness, Westboro Baptist Church, the kkk, etc.
But population wise. Isn't it that the majority of the USA population mostly mormons?