It is really astounding to think about it but the mainstream religions of America abandoned most religious parishioners.
I identify two movements in the church that brought about this collapse. First nearly every mainstream Protestant church joined Leftwing political activism, including anti-Vietnam War and general Lefty issues such as pro-union and strongly pro civil rights. Most church goers were not politically active. They went to church for religious reasons and were offended by turning religion into Lefty politics. Those who remained in the churches included mostly people who didn’t care about politics, like most of the general American public (half of Americans don’t vote).
The other movement that brought about this collapse was the churches distance from young people who were becoming hippies. Smoking dope and aligning with hippie spirituality. Churches never understood nor opened their doors to the new values of young people.
How do we know these were the movements that hurt the mainstream churches? First, the evangelical churches began growing rapidly at the same time. They stayed out of Lefty politics. They emphasized a simple spirituality with most efforts directed at community building with clubs for singles, motorcycle riders and family training.
Second, churches like Glide United Methodist in San Francisco that had rock music, contemporary singing and vague spirituality attracted young people. Glide was copied by many mainstream churches that attracted young parishioners.
Mixing religion and Lefty politics is a bad idea. The same bad outcome occurred in Reform Jewish temples when rabbis confused Lefty politics with Jewish teaching. Membership disappeared.