Absolutely, not officially. But we used to operate under Judeo-Christian values. That's fallen by the way-side, and it hasn't been a good thing for this country.
Other than a fraction of a % who were Jewish, most Americans were at least nominal Christians at the time of the Revolution, the Civil War, and well into the 19th century. The appearance of substantial numbers of Hindus, Muslims, Buddhists, and other so-called faith groups is a very recent event. So yes, the U.S. was an overwhelmingly Christian nation until recently. It is now following the same path to cultural suicide as Europe.