Mainline Protestants has been in decline for decades. How about since 1950s?
This is still news? Mainline Protestantism, once central in U.S. culture, keeps collapsing
https://www.getreligion.org/getreligion/2023/9/27/this-is-still-news-mainline-protestantism-once-central-in-us-culture-keeps-collapsing
Think the numbers peaked in the 1960s, probably the first half of the decade. Getting numbers for some denominations in that time period is a little complicated because of mergers, like what became the United Methodist Church and the Evangelical Lutheran Church in America. Giving reasons for the decline is not, IMO, one single thing.
Compromising on and rejecting Scriptural authority and teachings is part of it, and almost every denomination has had their own path. The two exceptions I know of are the Lutheran Church Missouri Synod and the Wisconsin Evangelical Lutheran Synod. That I know of, WELS never started down the compromise road, while the LCMS started but had a small but noisy split in the 1970s when they did a hard U-turn. The UCC was already well down the road toward rejecting Scripture by the mid 60s, while ELCA's path became confirmed with its 1988 merger of two major Lutheran bodies and a liberal splinter off the LCMS, and more theologically conservative congregations splitting off around that time and soon after.
I think there are general cultural trends in the US that have contributed to mainline decline. Mainline Protestant churches tend to be liturgical and US culture has drifted away from formal ceremony. Many/most mainline Protestant churches have strong centralized authority, while US culture has trended away from conforming to centralized authority. Fancy expensive buildings (Edifice Complex) and vestments don't play well anymore, and even suits and ties not so much. IIRC, the LCMS and WELS are declining very slightly, where other mainline Protestant churches have declined much more significantly, but in a nation whose population is growing, holding steady is the cousin of decline.
On the Evangelical side of US Protestantism, it gets fuzzy. On one hand, the Southern Baptist Church has declined some, while the Assemblies of God denomination has grown some. But what is harder to measure but growing, possibly significantly, is independent Evangelical churches. Much of that growth is transfers from denominations like the SBC, some from Catholicism, some from mainliners, and some who are new believers. Getting stats from independent Evangelical churches is not easy, and some don't really track some stats very closely.
