The U.S. is not a “proposition nation.” If it were,
(1) its advocates would actually say what the proposition is,
(2) you’d have to swear adherence to that proposition to be a U.S. citizen.
The first of these is rarely true, and the second one is not true at all.
For example, if the proposition is that “the Constitution is good,” then why are people who call it “a worthless scrap of paper,” etc., allowed to remain American citizens?
These days, the phrase “proposition nation” is a propaganda phrase used by our enemies to deceive us. That may or may not have been true in the past, but I am talking about the situation now. Thus the actual content of the phrase “proposition nation” is, “You can be an American citizen whether or not you agree with the proposition.” This is, literally, the opposite of what “proposition nation” actually means (if it means anything).
Some people may try to get cute and argue that the proposition is the meta-proposition that people should be allowed to believe any proposition they want. But the modern left is explicitly against this idea, and is imposing ever-increasing restrictions on the advocacy of propositions the left disagrees with. Yet, none of the people who yap about “proposition nation” advocate stripping leftists who stifle others’ speech of their citizenship.
In summary: Not only is the notion of a “proposition nation” nonsensical, but apparently none of those who push that phrase are sincere about it.