These churches, he argues, tend to preach a message that emphasizes pluralism (many different viewpoints out there), individualism (it's up to you to decide what to believe), and free and open intellectual inquiry (don't let creeds hold you back). These values, however, as good as they may be to certain degrees, tend to erode institutional loyalty and organizational solidarity.
So, somewhat paradoxically, the organizational decline in the church is the result of the message being received, not ignored. To put it simply, the culture at large has embraced these values that have been promoted by liberal Protestants, and this has now made the continued existence of liberal Protestant churches irrelevant. People don't need to come to church to learn that they need to be open-minded and tolerant, since these values are now largely embraced by the culture as a whole.
While I am sure that the decline of mainstream churches is due to more than simply this, I don't think this can be discounted as an important contributing factor.
I am all for churches being places of critical thinking and intellectual exploration. I have even tried to instigate a bit of this myself. I am also all for cultivating a church community that doesn't try to dictate the correct answer for every ethical and theological question that we have. We see through a glass dimly, and like it or not, we have to live with a good deal of ambiguity and uncertainty.
One of my favorite quotes from John Wesley is in his explanation of Methodism, where he is listing things that do not define a Methodist Christian. He says,
"But as to the opinions which do not strike at the root of Christianity, we think and let think."While Wesley endorses a wide and generous understanding of Christian faith, it is not an understanding without any boundaries. There is a "root" to Christianity that cannot be pulled up and it still be Christianity. For Wesley, the root of Christianity would no doubt include the uniqueness and supremacy of Christ as God's ultimate revelation of reconciling love.
Jesus is Lord.
Jesus is central to God's purposes.
Jesus is what God looks like in human form.
Jesus is the Savior of all humankind.
While we are certainly free to disagree with those statements, we are not free to disagree with them and still operate under the banner of "Christian." (We may have some doubts about them or be confused about what exactly they mean and still, of course, be a Christian, but we cannot positively deny them and still coherently and meaningfully claim Christian identity.)
Any religious identity must have borders or there is no ground for the identity in the first place. As Andrew Walker and Robin Parry put it in their new book Deep Church Rising: The Third Schism and the Recovery of Christian Orthodoxy,
"Orthodoxy may be a large tent but it is not infinitely large. Boundaries do need to be drawn and this, we maintain, is a good thing. If Christianity can be anything at all then it is nothing at all."
I'm afraid "inclusivity" has often meant watering down the gospel message so much that people can't find anything to disagree with. The problem with that approach is that you also then take away anything of substance for people to agree with and therefore, live for.
It's a shame that what we often get in churches is either a liberal church with no solid theological center, or a conservative church with way too many fixed theological boundaries. Wesley advocated a third way for his people: proclaim unity in Christ and in God's reconciling work through him, but don't expect uniformity. Wesley was all for inclusivity, but it was definitely a Christ-centered inclusivity.
Perhaps the way to sum up what I am driving at is this: our mainstream church emphasis needs to switch from Christ welcomes all, to Christ welcomes all.