Dear D.,
I suggest you have to begin with a more basic question: "Why has morality endured?" In the first chapter of Adultery: An Exploration of Love and Marriage, which Mollie and I wrote in Arabia, we point out that none of the ten commandments have any defining content. They are more like categories of moral judgment. All peoples of all nations of any religion or no religion discuss questions of right and wrong under these ten categories.
They all agree that murder or adultery or stealing or idolatry are wrong, and they know there must be some supreme being or creative principle, parents should be honoured, and covetousness avoided, but they give a different content to each of these moral principles.
At least it seems that the human animal is characterized by making moral judgments about himself and others, and loves to engage in moral discussion. It is hard to see why amoral evolution should throw up such a creature.
In the next chapters of the book on Adultery (it is not a how-to book!) we point out that Jesus filled out the content of the ten categories of moral judgment in terms of loving. And we define love as caring about the freedom of the other. But there again why should the survival of the fittest care about the freedom of others?
It is at that point that the Christian faith begins to make sense. If God is love and God's love cares about human freedom, and God want us to be perfected in love, then a model of the creative love of God (I call it Creative Love Theism) begins to make sense. It is then easy to show such a model can only work with a Trinitarian model of God rather than some form of Monism or Unitarianism (as in Islam or Humanism).
As long as there are people who are committed to love as caring for the freedom of the other, and then believing that this is what life and this universe is all about, Christianity will survive. The problem is that ecclesiastical hierarchies can't cope with human freedom, or even with love for that matter.
Bob.