This explains a lot

USA Today ran an essay Monday: Where have all the protestants gone?. Like many other essays of the sort, it wonders where all the mainline Protestants have gone as their denominations implode. It is an interesting question. If America is becoming more secular and less religious, why would denominations with modest requirements be doing so poorly while the most stringent faith groups flourish?

I think this captures it:

Religion professor and Presbyterian elder Peggy Cowan sees something else — something that could help churches connect with a generation of seekers who might consider portions of the Apostle's Creed on par with Harry Potter. "Theological exploration," she calls it. With Jesus as their tutor, successful churches in post-modern America must be willing to affirm that all theology is provisional in light of the mystery of God.

In that one paragraph you see the root cause of the demise of “mainline” Protestantism, i.e. abandonment of Biblical authority. You have a female elder who is trying to find a way to connect with people who think the Apostle's Creed is on par with Harry Potter and does so by basically assuming that all theology, no matter how foundational, is provisional in the light of the mystery of God. While our understanding of God is necessarily limited, there are many points that are revealed in sufficient detail to leave no room for doubt. There are areas on which we can disagree and still be brothers but we cannot chalk any difficult passages up to "mystery" to avoid obeying them. His sheep know His voice and His sheep also know when something is wrong. The voice of the Good Shepherd has been silenced in many mainline churches and as the elect flee and the unregenerate get bored, these denominations die. There is no mystery in it. A church that refuses to proclaim Christ and obey His Word ought to shrivel and die.