I say this as a veteran, a Christian, and a moderate: The United States of America is not now, nor has ever been, a "Christian nation." You could maybe call it a "Christian-majority nation" in the same way we refer to some nations as "Muslim-majority nations," but that's still a bit of a stretch given that all sects of Christianity, which includes denominations that other denominations do not consider Christian in the slightest, make up only 70%, and the single largest denomination classification according to Pew is Evangelicals, who largely have a problem considering the second largest denomination classification, Catholic, Christian at all.
I say the following as someone who was studying to become a chaplain, and thus studied a lot of biblical theology, church history, etc. Christian theocracy never functions well. If it functions for an extended time at all, it's so quickly corrupted and turned into something blatantly against the teachings of Christ as recorded in the Gospels that the only reason it's called Christian is to grant the people abusing their power more legitimacy. If the USA ever becomes a Christian theocracy, it will fall, and it won't be pretty. If that happened during my lifetime, I would gladly do whatever I can to follow through on my oath to protect and defend the Constitution against all enemies, foreign and domestic, and no matter what, theocrats are an enemy of the First Amendment and thus the Constitution.
And for those wondering where my loyalties would lie because I'm still Christian, Christ said to "give to Caesar (meaning the government) what is Caesar's, and give to God what is God's." Last I checked, ethical and theological truth are God's, and law, politics, etc. belong to Caesar. Allowing Uncle Sam or any of his little cousins (state governments) to call the shots ethically or theologically is taking from God what is God's and giving it to Caesar, a blatant violation of the order Christians were called to maintain. Both my identity as a veteran and my identity as a Christian are in sync: the United States will not become a Christian theocracy if I can do anything to stop it from doing so, and I certainly hope I am not alone in that. And if it somehow does, and I can do nothing to stop it, I will consider the United States of America to have broken faith with anyone who swore any oath to the USA under the Constitution, and I will not defend her again.
I hope I never have to follow through on those last bits. Like many who chose to become warfighters, I don't want war. I just know what to do with it if we have it.
God Bless if you'll let Him, and have an otherwise great day.