Yes, these churches are hemorrhaging members in startling numbers, but many of those folks are not leaving Christianity. They are simply going elsewhere. Because of this shifting, other very different kinds of churches are holding strong in crowds and have been for as long as such data has been collected. In some ways, they are even growing.
The Stanton article points out growth in nondenominational churches that are Evangelical in outlook, and states that these groups gain five new congregants exiled from the liberal churches for every one they lose for any reason. “They also do a better job of retaining believers from childhood to adulthood than do mainline churches.”
Over time, even as the “nones” increase their share of the population, at the expense of weakly affiliated religious people, the “strongly affiliated” have not only held steady but even grown slightly, and currently number about 40% of U.S. adults. One third of American adults pray multiple times a day, and believe that The Bible is God’s actual word. Stanton points out that church attendance today is higher than at the time of the founding of our nation.
The 2017 study in question is “The Persistent and Exceptional Intensity of American Religion: A Response to Recent Research “ by Landon Schnabel of Indiana University, and Sean Bock of Harvard (Sociological Science, 4, 686-700).