I wonder (and no more than wonder) if we're seeing some kind of romantic nationalism acculturated in an American context that requires it to take on the idiom and practices of evangelical Protestantism. That, to me, seems like the nearest description of what we're seeing.

I suspect this framing will have greater predictive power than if you take them to either be a natural outgrowth of American low-church Protestantism OR a departure from it in everything but name.