And to an atheist it's simply paying homage to a magic man (or woman) in the sky. It has no basis in reality, and it would be just as valid to declare the US to be one nation under leprechauns.
And that can be done without forcing said faith down the throats of everyone else in the country. Faith is a personal experience, not a community requirement. And while I can see plenty of good and generous people of faith around, I have seen damned few religions which are good and generous to anyone other than their own kind. And religions tend to use people's faiths to turn them against those who are different.It makes no difference so long as your faith promotes goodness and generosity to your fellow man.