I've been struck by a phenomenon that has hit Episcopalians, Presbyterians, Methodists -- all those denominations whose protestant Christianity defined most religion in America (hence the "mainstream denominations") -- and their common failure.
It's not hard to define. Each of them has recently sheltered two religions under their banner: one, recognizable from the past, and a recent one with radically different answers to the basic questions.
What are the basic questions?
Who is Jesus Christ?
What is the authority of the Bible for belief and practice of being a Christian?
How should people respond to God's call to live a holy life?
I'd recommend these questions to anyone who wants to explore the fault line. You will find two distinctily different sets of answers out there. In addition, you will find those who know that there are two religions sharing space and who decline to make a choice between truth and falsehood. You know that the church is failing once the keynotes become "dialogue" and "shared experience" -- neither of which is oriented toward truth, the basic job of the church. I've previously been "tolerated out" of places (don't mistake toleration for allowing diverse opinions to debate if you want to stay "in"), and I know what it does to congregations.
The main stream is drying up, folks. Time to find another source of water. God always supplies it.