Peter Leithart gives a clue. It has to do with ways of relating churches to the culture, coming along side it to use the vernacular of the Vatican, that would wind up devastating the Protestant mainline:
The growth that swelled the mainline during the 1950s was fueled by people looking for “a more relaxed, less legalistic, less dogmatic version of the faith.” Despite numerical growth, the mainline churches didn’t grow “stronger” during the 1950s; their grown “concealed an ongoing weakness that a few years later produced an unprecedently steep decline in membership” (194).
The authors see the drift in the mainline as an accommodation to cultural trends: “The American cultural climate has shifted during the twentieth century in the direction of greater relativism and skepticism in matters of religion, and toward greater degrees of individualism. Acceptance of diversity in belief, lifestyle, and ethnic and racial background has broadened markedly.” Initially promoted by elites, the shift became popular, and “the leadership of the mainline Protestant churches accommodated the shift within their own ranks.” When the Sixties hit, the mainline Protestant churches were already sailing with the same wind that carried the sexual revolution and the challenge to settled authority: “The mainline Protestant churches did not initiate the new shift, but they were unable and unwilling to resist it” (198).
Not surprisingly, Presbyterians lost the next generation: “The children have asked over and over what is distinctive about Presbyterianism – or even about Protestantism – and why they should believe it or cherish it. The answers have apparently not been very clear. Today Presbyterians should not bemoan the lack of faith and church commitment exhibited by their youth, since they have no one to blame but themselves. No outside power forcibly pulled their children away from the faith”
And what happened to the mainline in the 1960s, happened to Roman Catholics in the 1970s once the bishops at Vatican II opened the windows to modern society and hoped for a more relaxed church. (By the way, it could happen to all the folks inspired by TKNY. Some think it already has.)
Once again, it’s the progressives who pave the way for “progress” among Roman Catholics.
Why is it that the more you try to make Christianity relevant, the less Christianity you have left?