The wreckage of the former "mainline" denominations sits atop a modernist theology that began to dominate mainline seminaries during the early 20th century. Eventually, many mainline theologians came to reject nearly every doctrinal aspect of traditional Christianity. The resulting secularization of the mainline denominations has proven itself a self-limiting process; as these churches become further removed from doctrinal Christianity, they are being replaced by churches that offer more vigorous spiritual messages.
Even though so many have left, those remaining in the mainline pews still overwhelmingly regard the traditional tenets of Christian belief as central to their faith. Their exodus will continue as long as mainline clergy and seminaries claim it is their right to instruct the faithful in more "progressive" spiritual and political views.
The progressives/modernists/whatever are also comprehensively failing to pass on any understanding of why religion is important to their children, which leads to hemorrhaging on the left of the mainline denominations as well as on the right.
There is a certain parallel between the decline of mainline Protestant denominations in the West and the decline of social democratic/labour parties in Europe, which incidentally shared overlapping demographics in places such as the UK and Germany. In both cases, constant watering down of core principles and doctrines made adherents/voters see little point in continuing to supporting them while making the leap to secularism/conservative parties all the more easier. Moreover, their beliefs undermined key institutions that ensured reproduction of support-liberal theology made it less likely parents would teach their children Christian beliefs and the turn to the Third Way undermined labour union strength.
This is an extremely silly parallel. Social democratic parties didn't decline because their voters were so ideologically socialist that they reacted to Third Wayism by becoming conservative (how does that even make any sense?), nor because social democratic parties undermined labour unions (one could argue this in the case of idk New Zealand, but the UK? Sweden? France?). The decline happened because of the transition out of the industrial economy and the increasing fluidity of class society. Longue durée trends or tectonic plates shifting underneath Western society in general and not reducible to a voluntaristic lack of ideological will on the part of every single centre-left party in the world at the same time.
Anyway I'm not competent to comment on the membership of US mainline Protestant churches, and don't mean to dispute the lack of will thesis
for churches - it might be right for all I know. But it seems to me the proper comparison would be not be between churches and political parties because 95% of voters for a party aren't members and don't do anything for it except put a cross in a ballot box once every few years. The more proper comparison would be between churches and unions as similar institutions. The key driver of the decline of social democratic and labour parties: why has union density fallen? One reason that links to church membership is that people move jobs, and therefore communities, much more often than they used to 70 years ago. It's more difficult to maintain local institutions of every kind (e.g. Putnam's famous bowling clubs) when people are more transient, and transience applies doubly so to urban areas, where more people live now. Were the members of mainline churches more likely to move communities? Were the churches themselves based in communities which had greater changes in demographics than others? IDK I just made that hypothesis up and I don't know the answer. But it sounds more fruitful to me than bogus analogies to European political parties.
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