Minor nitpick: it was time to start worrying about Christian nationalism over twenty years ago. They've been trying to legitimize Christian nationalism since at least the founding of the "Moral Majority" when I was a boy. I've been trying to sound the alarm about Dominionism and Christian nationalism since the second Bush administration.
But if you're late to the game, fine. The second best time to start worrying about Christian nationalism is now.
As if these people care about a treaty they've never heard of and will never inform themselves about
That's why you ask them about he founding fathers first, then you can show them who signed it.