I think you're seriously underestimating the decline of participation in organized religion in the US both during the second half of the 20th century and in the twenty years since. Here are just a few numbers to chew on from Gallop's long-running polls[1]:
* The number of religious "Nones" - people identifying themselves with no religion - was right around 1% through the 1950s. In the 1970s it increased from 3-7%, and stayed below 10% through the end of the century. It's now over 20%. In the 1950's, ~70% of people were Protestant Christians. Now it's half that amount.
* In the year 2000, 12% of Americans reported that religion was "not very important" in their lives. Now it's double that amount.
* In the year 2000, 13% of Americans reported that they "never" attend religious services. Now 29% never attend.