There are no 'atheists' or 'agnostics'
First, it's doubtful how "Christian" a country America was to begin with. Countries don't have religions; people do. And if there has ever been a time when a majority of Americans were serious disciples of Jesus, it's a long time ago. More to the point, we as a people may recognize that we are sinners in principle. But as a practical matter, being one's own savior is the American way. We like to pull ourselves up by our own bootstraps (or tell ourselves that we have) in the spiritual realm as well as in all the others. The theology of most of the popular American religious movements leaves little doubt of that, nor does our national religious history.
For most Americans, religion is pretty much what President Obama and the political Left seem to think it should be: something private, personal, and easy, involving no necessary sacrifice and having particular impact on one's behavior. Certainly, for most of us, it can involve nothing which might bring one into conflict with others. Religion, like politics, is something best not discussed. And increasing, religion is something Americans are even more ignorant about than they are about politics.
There was a time, at least, when churchgoers were biblically literate. At least marginally.
No, for Americans, religion is something to be trotted out for marriages, baptisms, funerals and other family occasions, to provide an increasingly rough structure for loose, pliable, and above all individualistic moral boundaries, and to provide often sappy, banal pseudo-comfort in times of trial through the trotting out of ill-understood but vaguely comforting clichés.
The second problem with the poll's findings is that there really is no such thing as an "atheist" or an "agnostic." Everybody has someone or something which he or she values above all else. Everybody has someone or something to whom (or to which) he or she turns when the chips are down and all else fails. Everybody has a "bottom line-" someone (or something) which is, without fail, to be "feared, loved and trusted above all things," in Luther's phrase.
I believe that in fact a majority of Americans- including those who claim to be atheists and agnostics, and even most of those who claim to be Christians or to adhere to some other religious tradition- in fact have the very same god. Rachel Held Evans's recent piece in the Washington Post in which she combines an admirable appreciation for historical liturgy with the plea for the Church to attract the Millennial generation by selling out to the culture on matters of sexual ethics is a perfect illustration of that dominant American faith in action.
And here is a video sample of what would be in that faith's hymnal, if it had one:
Let it be noted that the Christian God- unlike this one- doesn't allow us to make our religion up to suit ourselves as we go along.
Or to suit society or the government, either- or certainly the demands of political correctness.