Sometimes we get a whole nation committed to Christ governmentally, such as America used to be,
No, it never was. That's your own revisionist history right there.
You don't seem to understand that if America had been a Christian nation (in the sense that you mean), then it would have been Catholic or Episcopalian. It was the non-establishment clause of the first amendment that allowed Evangelical Christianity (and crazy creationist cults) to flourish.
Fundamentalism - the anti-American, anti-Christian branch of American Christianity