One of the major findings from the Pew Research Center's recently-released update to their American Religious Landscape Survey is the decline of Catholicism (down 3.1%) and Mainline Protestantism (e.g. Methodism, Presbyterianism, Episcopalianism; down 3.4%) among the American population compared to Evangelical Protestantism (down only 0.9%) over the last seven years.
A popular explanation for the decline of Mainline Protestantism is that it has compromised its doctrines and beliefs by "watering itself down" and becoming more theologically progressive and politically liberal, with a strong emphasis on inclusivity and social justice. In other words, the Mainline Church is dying off because it has gone "astray" from core Christian teachings and beliefs to the point where it is increasingly irrelevant among religious Americans. (See here, for example.) The take-away lesson for Christian denominations that want to thrive: Don't give in to secular trends! Stay true to your conservative orthodox beliefs and practices!
As compelling as this explanation may be to some, there is an alternative explanation that is worth considering.
In their landmark work on American religion and politics, Putnam and Campbell (2012) argue that the strong link between Evangelical Christianity and political conservatism in the 1980s and 1990s has been a key factor in the growing disenchantment of the Millennial generation with Christianity (and organized religion altogether) throughout the 2000s and 2010s. The explanation goes something like this: Millennials tend to be more politically liberal and identify more strongly as Democrats. They come of age and see an American social environment where being religious also usually means being a conservative Republican and vice versa. Young people see the Religious Right and think "if being religious means conservative Republican politics, then being religious isn't for me." Putnam and Campbell further argue: "Continuing to sound the public trumpet of conservative personal morality may be the right thing to do from a theological point of view, but it may mean saving fewer souls now than it did a generation ago."
In this context, the recent rapid decline of Mainline Protestantism revealed by the Pew survey is likely NOT due to its move in a more liberal direction. On the contrary, Mainline Protestantism has declined because the link between Evangelical Protestantism and conservative politics have created an environment where young liberals (and many others) feel that they have to choose between their political identity and a dominant cultural religious identity that has only conservative political and theological expressions.
In other words, Mainline Protestantism is not declining because it has liberalized. It is declining because its key demographic constituencies have been implicitly forced to choose between their religious and political identities in a way that Evangelical Protestants are not. Being forced to choose, their political identities (so far) have won.
To put it more bluntly: Liberalization is not what is killing Mainline Protestantism. The intensive three-decade fusion of Evangelical Protestantism and political conservatism is what is killing Mainline Protestantism.
This explanation suggests that the way to prevent further disenchantment with organized religion among young people is for religious leaders and laypeople alike to sever the exclusive link between religion and conservative politics in the United States. Foster an environment where Christianity is freely and confidently expressed through both political conservatism and political liberalism. Then those liberal-leaning Millennials (and generations after them) will not be forced to choose between their political beliefs and their religious aspirations. It is possible that current trends may slow or even reverse. Many would find a natural home in Mainline Protestantism, and institutional religion, once again.