Is the USA a “Christian nation”?

There is no such thing as a “Christian nation.” The Bible says that the Lord establishes all nations on earth according to His purpose:

Acts 17:24 “The God who made the world and all things in it, since He is Lord of heaven and earth, does not dwell in temples made with hands;
Acts 17:25 nor is He served by human hands, as though He needed anything since He Himself gives to all people life and breath and all things;
Acts 17:26 and He made from one man every nation of mankind to live on all the face of the earth, having determined their appointed times and the boundaries of their habitation,

The Lord has determined the time and place for every human to exist on earth. The Lord designated one nation above the rest, Israel, and then He created all Gentile nations without distinction. The Lord created those divisions and assigned us as He chose, and He says that no Gentile nation is any more godly (or Christian) than another. 

In fact, the Lord says that all men (whether Jew or Gentile) are sinners with hearts that are far from Him:

Rom. 3:9 What then? Are we better than they? Not at all; for we have already charged that both Jews and Greeks are all under sin;
Rom. 3:10 as it is written, “There is none righteous, not even one;
Rom. 3:11 There is none who understands, There is none who seeks for God;
Rom. 3:12 All have turned aside, together they have become useless; There is none who does good, There is not even one.”
Rom. 3:13 “Their throat is an open grave, With their tongues they keep deceiving,” “The poison of asps is under their lips”;

So the belief that the United States is a “Christian nation” is nothing more than American Christian self-righteousness combined with political mythology. Ironically, many of the United States nation’s founding fathers (e.g., Jefferson, Franklin, Madison, etc.) were self-avowed atheists or theists, not Christians. Even the faith of puritans or others quasi-Christian pilgrims is unclear.  

Regardless of the nature of their faith (or lack thereof), we simply cannot credit an entire nation of people with saving faith based on the actions or beliefs of a few. As Paul says, only those who have the indwelling of the Spirit of God by faith are part of the Church:

Rom. 8:9  However, you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. But if anyone does not have the Spirit of Christ, he does not belong to Him.

In summary, the United States is merely a Gentile nation like any other on earth: one occupied by millions of sinful, lost and dying unbelievers together with some born-again Christians.