The main forms of immigration were Irish (very Catholic) Italian (very Catholic), German (Strong Protestant) and Hispanic (extremely Catholic). African American communities also have a very strong faith element. As do many Asian communities there. Add this to a society formed by the puritanical and you end up with America.

Faith has always been strong among the dispossessed and downtrodden, it was often all they had in the way of hope and to bind them as a community. It's why slaves placed such importance on faith - it was their only hope (and that's another essay).

We tend to think of America as the cosmopolitan East and West Coast cities which dominate their cultural output - the reality is they are very different from Middle and Southern America which are enormously God fearing, even today.