As far as I understand the recent decline in the numbers is that people stopped lying about which church they don't go to. Because church-goer numbers are stable.
What happened during the last century is ... complicated. For one thing religion was never really that organized in the US. (Maybe except in Utah. But that's also relatively new.)
I think simply WWII, and the post-WWII economic boom (plus the GI bill), plus then the heating up Cold War slowly but surely transformed society. For the new generations the various Christian belief systems offered by churches were simply not a real option.
People got an appetite for different answers whether be that science or pseudoscience based. Cults and other ideology-based groups filled some of the vacuum. (And of course the counterculture eventually and then after Vietnam and the race riots came the backlash. The Southern Strategy, which platformed evangelicals, but as a political group, not as organized religion. Basically emptying out the spiritual part, etc. And of course it still works.)
What happened during the last century is ... complicated. For one thing religion was never really that organized in the US. (Maybe except in Utah. But that's also relatively new.)
I think simply WWII, and the post-WWII economic boom (plus the GI bill), plus then the heating up Cold War slowly but surely transformed society. For the new generations the various Christian belief systems offered by churches were simply not a real option.
People got an appetite for different answers whether be that science or pseudoscience based. Cults and other ideology-based groups filled some of the vacuum. (And of course the counterculture eventually and then after Vietnam and the race riots came the backlash. The Southern Strategy, which platformed evangelicals, but as a political group, not as organized religion. Basically emptying out the spiritual part, etc. And of course it still works.)