The term "Christian nation" is ambiguous. It could simply mean a nation that has been significantly influenced by Christianity throughout all or most of its history. If that's all one means by "Christian nation" then it's not problematic, but I think those who use the term often mean more.
It could refer to a nation (like the U. K.) that has a state-sanctioned Christian church. A nation like this could be considered at least nominally Christian.
A "Christian nation" might also simply be a nation where the majority of the citizenry are Christians. This isn't as simple as it seems, however. Does one include all professing Christians? If not, then one must determine what constitutes true Christianity. This is setting aside the obvious problem of changing demographics which means that, theoretically, a nation could be Christian one year and secular the next.
It seems to me that most use the phrase to mean something like a combination of these three definitions: A nation where Christians make up the majority and which has had significant Christian influence including some sort of official recognition up to and including an official state church.
The first meaning is obviously true (the U. S. has been significantly influenced by Christianity). The second meaning is obviously false (the U. S. does not have a state church). The third meaning could be true or false depending on how one looks at it. Nominally the majority of the U. S. population is considered Christian, but it's debatable whether or not that's ever actually been the case.
This raises more questions: What level of official approval or recognition, if any, has Christianity enjoyed throughout U. S. history? Were the founders Christians, deists, or a combination of both? Were our founding documents influenced by Scripture, pagan and secular sources, or some combination of the two? Were Christians in the majority during the nation's founding? Are they in the majority now? Should we include all professing Christians or use additional criteria when counting the number of Christians?
So, the question of whether or not the U. S. is a Christian nation, from a historical perspective, is a difficult one.
What about the theological perspective? When some refer to the U. S. as a Christian nation they are not merely making a historical claim but a theological one.
They might mean that God was providentially involved in the founding of the United States. This is true but trivial. A biblical understanding of providence requires us to affirm that God is involved in the founding of every nation (and everything else), so of course He providentially directed the founding of the Unites States. He was just as involved in the founding of North Korea and Iraq, though.
Another theological claim made by some is that the United States is a covenant nation. This is utter nonsense. There are several problems with this claim:
- The only nation that God ever entered into a covenant with was the nation of Israel (not to be confused with the contemporary nation of Israel which shares a name with the biblical people of God and nothing else).
- Those who claim that the United States is in covenant with God clearly do not realize what that would entail. Did George Washington ascend Mount Sinai and meet with God face to face? Did God write the constitution on tablets of stone with his own finger? Where is the third testament of Scripture including the gospels of Washington, Jefferson, and Franklin?
- Even if the founders of the U. S. had wanted to enter into a covenant with God, they couldn't have. A covenant requires two parties. One cannot simply unilaterally impose a covenant on the King of the universe. God is the initiator of his covenants with man. He sets the terms. One cannot simply decide to cut a covenant with God.
- If men want to be in a covenant relationship with God, they must go through Christ. The New Covenant of which Christ is the head is entered into on an individual basis and includes people from every tribe, tongue, and nation. The idea, then, of a "Christian nation" is incompatible with the New Covenant. The continuation of the people of God is found in the Church not in the United States of America.