The United States is not and has never been a Christian nation. My Southern Baptist friends, people who actually think about literary criticism of the Bible, do not believe the "government of men" of the United States has anything to do with the "kingdom of heaven". The idea that somehow the United States has anything to do with Christianity is somehow offensive to both the United States as well as Christianity — and I don't know who ought to be more offended, Americans or Christians.
Claude:
This is a genuinely interesting double-edged critique, and it cuts cleanly in both directions.
The American side of the offense:
The Founders were, at best, a mixed bag theologically. Jefferson literally cut miracles out of his Bible with scissors. Franklin was a deist who thought God had better things to do than micromanage republics. Madison — the architect of the Constitution — was arguably the most emphatic of all: the First Amendment and Article VI's "no religious test" clause weren't accidents or compromises, they were design choices. The whole structure presupposes pluralism, not Christendom. Calling America a Christian nation flattens this deliberately secular architecture.
The Christian side of the offense:
Your Southern Baptist friends are actually in good theological company going back centuries. The Anabaptist tradition, Augustine's two cities framework, Luther's two kingdoms doctrine, and certainly the more rigorous Calvinist traditions all maintain a sharp distinction between earthly governance and sacred covenant. Conflating a constitutional republic — built on Lockean social contract theory — with the Kingdom of God isn't just historically sloppy, it's arguably theologically dangerous. It risks making Caesar into Christ, which is precisely the idolatry the tradition warns against.
The irony at the center:
The "Christian nation" claim tends to be weakest on both the history and the theology simultaneously. It usually rests on cherry-picked Founder quotes stripped of context, plus a flattened read of Scripture that ignores exactly the literary-critical sophistication your Baptist friends bring to the text.
So who should be more offended? Probably Christians, on net — because the claim is simultaneously a bad historical argument and a theological mistake that domesticates transcendence into nationalism. Americans can at least point to the Constitution and say "that's just not what this document is." Christians have to also argue against something that actively distorts their own tradition from within.
The Treaty of Tripoli: Signed in 1797 under President John Adams, this official diplomatic document explicitly states that "the Government of the United States of America is not, in any sense, founded on the Christian religion."