Christian Nationalism is un-Americann
“Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof [...]"
Depending on the implications of "preserving and promoting Christianity," I think Christian nationalism may or may not be unconstitutional. Politicians can and have justified a lot of political policies ex post facto by saying they "uphold Christian values" (anything from helping the poor to banning abortions). That doesn't automatically make the policy unconstitutional though. Additionally, amending the constitution is itself constitutional, which makes designating things as unamerican this way a bit fraught. And even if we could, I'm not sure American-ness is a great proxy for policies being good or bad. Slavery was at one point American and constitutionally protected.
And all that being said, I probably agree that separation of church and state is necessary to avoid a lot of problems. But not due to it being American; I think that's dangerously close to trying to get an ought from an is.