This is historical and sociological theorizing about the numerical fluctuation in Christianity (Catholicism, Protestantism, and Evangelicalism) in America.
See Brad East's reflection about the decline of mainline Protestants. See link below.

Then Paul Vander Klay.
https://x.com/PaulVanderKlay/status/1925913677833593078?t=gzQV7-2QhZhjYrq3qXJtfA&s=19