The author in this book analyzes how the social, political and religious cultures of the United States interact and reinforce one another. America has been a predominantly Protestant nation since its founding, and so for most of that time a Protestant consensus about social and political behavior provided the common language for our national discourse. Since the 1970's the old "Mainline" Protestant denominations (Presbyterian, Methodists, Episcopalians, United Church of Christ, etc. - the denominations that have been the main membership of the National Council of Churches) have been in steep decline, losing large percentages of their memberships, and also the political and cultural influence they once had. The rise of the "Nones" - the people who claim no religious affiliation - has gone way up, although many of them will claim to be "spiritual but not religious". These people he describes as "Post-Protestants" since they tend to come from families that were once in the Mainline Protestant denominations.
This decline thus produced a vacuum into which came a strange alliance of Evangelicals and Roman Catholics. The second half of the book discusses the evolution of Catholicism over the past half century, both as an institution and as a set of ideas. The alliance of Catholics and Evangelicals has not been particularly successful in creating a new political and moral consensus. Witness the current divisiveness in our politics.
There is so much more that could be said here, but all I will say is that this is a book that should be read by anyone seeking an understanding of the internal challenges our nation faces.