It's a complex question. Joe Bageant, author of Deer Hunting With Jesus, can be a good explainer of evangelical religion in America to the rest of us. He assert

It's a complex question. Joe Bageant, author of Deer Hunting With Jesus, can be a good explainer of evangelical religion in America to the rest of us. He asserts that there have always been people who are this deeply religious in about this proportion, but the difference is that now they are a political constituency and a valuable demographic for advertisers. Perhaps it's very simply that culture isn't controlled by the city anymore, nor by the three big TV networks who all had similarly liberal urbanites in control. If I were to guess I'd say that what's on the decline is nominal religiosity. Those who don't believe, don't worship, in greater numbers. At the same time, among those who do believe, there's been an assertion of cultural power since the 1980s or so. No data here, just an observation from someone about the same as as pg.