It sort of depends on what you mean by “Christian nation.”
Almost everyone is a Christian? The state confesses itself as a Christian government? Christianity the official religion? Christianity taught in schools? No, so in these ways we are not a Christian nation.
Founded by almost exclusively Christians? Founding documents and history either explicitly or implicitly Christian, at least in values but often far more? A history of official Christian teaching, worship and standards in our public schools? Bible verses all over, Christian art, Christian quotations? A huge percentage of the population claiming to be Christians? The largest missionary sending nation in the world? The most charitable nation in the world? A system of laws and government that is largely Bible-based? Although less in recent decades? Yes, then we are Christian nation.
So, you have to define your terms.
We’re not so much a “Christian” nation as we are a “Zombie” nation.
Cue the music.