From the (scholarly?) Wikipedia:

Postchristianity is the decline of Christianity, particularly in Europe, Canada and Australia, in the 20th and 21st centuries, considered in terms of postmodernism. It may include personal world views, ideologies, religious movements or societies that are no longer rooted in the language and assumptions of Christianity, at least explicitly, though it had previously been in an environment of ubiquitous Christianity (i.e., Christendom).

For nearly two thousand years, the dominant thought in the Western World has been Christianity. Christianity made sense of a world that the earlier Greek philosophers had tried to describe. Christianity took the stagnating Greek cosmology and natural philosophy and brought it into an era of enlightenment on the basis of God’s continuity and order.

Christianity provided the way forward for philosophy, art, and theology. It gave birth to science, the empirical study of a natural world assumed to be ordered on the basis of the order of God. Christianity provided security, morality, ethics, and more.

And in the past 112 years, we’ve somehow developed societies “no longer rooted” in it? I find that very difficult to believe. It is the same difficulty with which I could see a 90 year old man suddenly forgetting his past by choice in order to embrace a totally different reality for the last 10 years of his life. Whereas the elder may simply be senile, the society is simply ignorant. Forgetful perhaps. After all, we have more important things to worry about here in sophisticated 21st Century Western Civilization. Things like Science™. iPods are Science™. Dawkins is Science™. Contradictory statistical studies for medicine are Science™. Christianity though? Not Science™. Not if you want to fit in, anyway. But we’re not about fitting in here; we’re about historical accuracy, and Postmodernism is a great way to get a society to swap the two goals.

From a blog I frequent:

“The dominant philosophy of the modern age is a deconstruction of, or, to be blunt, a hatred of love of truth. Modern ethics is relativism, the denial that ethics exists; modern aesthetics is subjectivity, the denial that beauty exists; modern logic is polylogism, the denial the man of different races, classes and backgrounds can reason together; modern ontology is materialism, the denial that thoughts (including thoughts about ontology) exist; modern epistemology is empiricism (a theory which by definition can enjoy no empirical support); modern metaphysics is nihilism, the belief that the truth is a formless void on which the human will writes whatever it wills. Modernism hence denies and deconstructs philosophy at every point. And modern theology is a muddy atheism lacking even the fire and dignity of Nietzsche or Celsus.”

Perhaps “Postchristian” is simply a way to distinguish those who are not interested in history.