It doesn’t seem like this was always the case - obviously there’s a lot of myth making about the “founding fathers”, but it does seem that a lot of them were genuine Enlightenment men.
I’m not under any illusions that the USA was ever a secular nation, but it seems like the phenomenon we see now, of right wingers marrying America = Christianity, Christianity = America, in their worldview, wasn’t always there.
Is it just the result of Cold War propaganda, juxtaposing the American empire of Christendom with the evil atheist soviets?
One of the big ironies with the religious fervor in America as opposed to Europe, which actually explains it to an extent, is that America lacks an official state church while European states typically do. So while the inherently bureaucratic nature of those state churches led both to a grip on the religion of the country while also causing the populace to treat it like any other bureaucracy, in America the lack of a state structure created a much more small business owner approach, especially as the nation was largely Protestant. Which led to, as comradeda put it, individualist pastors who put an emphasis on their own charisma and convincing of the flock to be dependent on them as individuals to receive the word of God. Competition for flocks lead to greater emphasis on the importance of the narrowness of the understanding of the dogma. Hence the tendency for splinter sects, slight differences in interpretation being treated as heresy, and pastors needing to maintain control over the lives of their flocks. All of which leads to greater religiosity.