While we remain a nation decisively shaped by religious faith, our politics and our culture are, in the main, less influenced by movements and arguments of an explicitly Christian character than they were even five years ago. I think this is a good thing—good for our political culture, which, as the American Founders saw, is complex and charged enough without attempting to compel or coerce religious belief or observance. It is good for Christianity, too, in that many Christians are rediscovering the virtues of a separation of church and state that protects what Roger Williams, who founded Rhode Island as a haven for religious dissenters, called "the garden of the church" from "the wilderness of the world."
In that Newsweek article, R. Albert Mohler Jr. defines Post-Christianity as "spirituality, however defined, without binding authority". It's spiritual anarchy.
Since the Reformation, we've had a constant chorus of voices singing the praises of new sects (and in a few cases, religions). Typically though, those sects found common physical as well as spiritual ground. Look at the American colonies for example. Massachusetts was home to the Puritans. Pennsylvania was home to the Quakers. Most of the southern colonies were Anglican, at least until the Great Awakening in the mid-18th Century. Even Rhode Island, beacon of religious liberty to Meacham, had plenty of "blue laws" less than a century after its founding in 1636. Rhode Island is actually a perfect example of differing sects embracing the idea of "toleration", while continuing to try and seek common ground when possible.
Back then, you also had the influence of Deism on virtually all Christian denominations. Deism, despite what modern pundits would have you believe, is not akin to agnosticism or atheism. Deism still puts God at the top of the pyramid, scripture is still worth reading, but Truth can also be found outside of the Bible. Contrast that with the Me-ism that is gaining ground today, and Deism starts to look as fundamental as an Assemblies of God congregation.
We've given up on the Bible as a source of Truth, and we've also decided to ignore the vast collection of wisdom acquired through the centuries by secular scholars and philosophers. Instead, we tell ourselves that all is a matter of perspective, that there is no Truth to be found (or, if there is Truth to be found, we already possess that knowledge somewhere inside of us). The result is that too many of us are spiritually adrift, floating aimlessly through life without ever really seeking Truth.
Finally, by becoming Post-Christian, we are also in danger culturally of being Post-Western Civilization. Christianity, in all its incarnations, is woven throughout the intellectual fabric of our shared history. It did not influence Plato and Cicero, but philosophers like Grotius (the father of international law) were influenced by secular minds like Aristotle as well as religious minds like St. Augustine. You cannot separate Christianity from the rest of history, and when Christianity is seen to taint all it touches, we throw a lot of valuable knowledge away.