This is very similar in my mind to scaffolding:
1. a temporary structure for holding workers and materials during the erection, repair, or decoration of a building.
Once a building is complete, the scaffolding is removed. If that means nothing to you, think instead of training wheels. Once I could ride a bicycle unassisted, my dad removed the training wheels.
What's my point? I think it is the same with America's Christian/moral heritage. Some argue that since America was built with Christian morality that America will collapse without it. But this argument falls logically short. Is Christian morality America's foundation or is it merely scaffolding? Is it a wheel or a training wheel? See, you can argue all you want about how America started, but it does not logically follow that America must be the same way today.
If money has worth because the government says so, cannot also America exist simply because the government says so? The analogy with currency is particularly interesting given the current economic situation. It has been suggested by some that we return to a gold standard to solve the problem. If that's the case, it would be an example of mistaking the foundation for the scaffolding and the consequences coming to haunt us. And it seems probable we'll make the same mistake with morality.
Even so, my first allegiance lies with a King and his kingdom, not America.