It is the loss of Christianity which is at the root of Western cultural decline, and the essence of decline, and the essence of decline is that peculiar mixture of hedonism and despair, incoherent glorification of emotions and bureaucratic rationalism which is New Leftism or Political Correctness.
But people generally misunderstand the causality in this analytic claim, people generally assume that it is the loss of an authoritative Christian moral framework which is the primary problem. Or that the claim is that self-identified Christians are more virtuous than agnostics or atheists. People assume that the basic problem is that our culture can no longer state what is right and wrong and underpin statements with divine authority - can no longer say do this and don't do that because God says so.
But these are not the essence of what went so deeply wrong when Christianity was lost from Western public discourse.
The essence of the problem is metaphysical. Metaphysics refers to the basic structure of understanding reality. The bottom line about how things are. It is my conviction that differences in metaphysics or changes in metaphysics always come out, and reveal themselves in multiple ways - for example they set the tone or flavour of a religion, the focus of it, the emphasis.
What was lost with the decline of Christianity was a universe, a basic reality, in which good and evil were real and objective, truth and lies were actual, beauty and ugliness... When Christianity was deleted from public discourse, then virtue was no longer seen as being a thing built into the universe, but only a state of people's minds and people's minds as being only a temporary arrangement of stuff - often changing and terminated by death.
This is what I mean by a change in metaphysics. This is far more profound a change than a change in the shopping list of good and bad, modifications or reversals of the Ten Commandments. Because when the modern metaphysics is established then even if there was no change in the Ten Commandments then they have become merely a thing existing - temporarily, and indeed arbitrarily - in the ephemeral minds of men, and utterly disconnected from permanent reality.
So, to call oneself a Christian, sincerely to regard oneself as a Christian, while holding this faith within a context of mainstream modern metaphysics, is not to be a Christian at all, not really. It is just to regard Christianity as a subjective, ephemeral, lifestyle option within a primary metaphysical worldview that renders Christianity trivial.
No matter how much a self-identified Christian may say this or that, or do this or that, it does not matter one way or another unless they have repudiated the mainstream modern public secular metaphysical discourse.
And to do this means to live in a state of internal exile, to be fundamentally misinterpreted and misunderstood, and to be unable to communicate concerning everyday matters of fact and motivation.
Unless the conversation can first be placed on a metaphysical level, then communication is not possible, and if there is a refusal to consider metaphysics, then there can be no communication, but merely misunderstanding of one sort or many sorts.