The amazing thing to me, as I study history, isn't that Rome fell, but that the Germanic barbarians were the only ones who seemed to care by the end. The Romans of Roman stock were more worried about taxes and money, while the German nobles wanted to preserve, and ultimately be, Romans. That is why Christianity survived, because by the late 5th century the barbarians were Christian an viewed it as something to preserve. Even though many where some flavor of Arians, they were very sympathetic to the Church, and ended up converting because that is what the Roman's are.
Such will not be the case this time. The muslim hordes who are spreading across Europe have little desire to preserve the culture or religion. The fall of the West will be more like the East this time. With minarets in St Peter's square. In such "Catholic" countries as Spain and Italy, the next generation will not be Christian. In the North, the old bastions of Calvinism and Lutherans are already fading away. Even here, the State bends over backwards to accommodate islam while snubbing Christianity. A remnant will be saved, but they may not survive the coming night in the West.
But Africa and Asia are having the Word spread like wild fire. There are majority Lutheran countries in Africa, and more Christians in China than in Europe or the US. The Church will survive, though we may not recognize it in a generation.
As you point out the West predates Christianity by many centuries. I do look back on both Rome and Greece as part of my heritage and inheritence, despite being neither Greek or Roman. And the Western Slavs, from whom I am descended, did not convert to Christianity until forced to by the Tuetonic Order as part of the Norhtern Crusades.
I desire the West to survive. Christianity was central to the west for about 1400 years, and many of our greatest achievements were inspired by it.
None the less it now seems to have not only lost it's force here, but in many cases be an active agent of our destruction. One need only look at the many Christian organizations that are pro-homosexuality, pro-amnesty, anti-American, anti-Israeli, etc.
Perhaps that is needed in China and Africa (both notoriously repressive and prejudiced cultures)? Maybe we need something different now, just as Hellenistic Paganism was no longer relevant at the start of the Dark Ages, perhaps Christianity as we understand it today isn't relevant for the new Dark Age of the West that it has helped usher in.