Christianity as Answer to Paganism
It came to me when I was reading pagan epics -- Gilgamesh, Homer, et al -- for a comparative literature class many years ago. A central theme of the epics is the potential nobility of the human condition that comes from striving to lead an exemplary life in the face of inevitable death. This aspect of the human condition separated men from the gods who didn't have to face death. There was something ennobling about the struggle with death. And the gods, being immortal, couldn't partake in it. The epic hero, despite his shortcomings, was more courageous than the gods and, in a way, morally superior to them. This I think is the core appeal to the epic literature, the courageous striving in the face of death and stoically dealing with the capricious actions of the gods.
An interesting period of history, and perhaps not studied enough, is the time between the decline of Greek civilization after Alexander and the rise of the Roman empire. It is a time when we have a meeting between Greek and Semitic cultures. Out of that brew came Christianity.
So here's the thesis: Christianity was the Semitic answer to paganism. Semitic culture had some appeal to the Greeks, it's mystery, it's monotheism and so forth, but it was quite culturally bound to the Jewish peoples. It's God was remote, tribal and not very approachable to outsiders. The Christ story brings God down to earth to face death and He becomes like an epic hero. God humanizes himself and becomes very personal. This greatly enhances the appeal of Semitic religion. God is ennobled by facing death and becomes more worthy of worship.
If I recall correctly, Soren Kierkegaard touched on this and remarked that the incarnation was the most important aspect of Christianity. He also thought the resurrection was of dubious value and actually defeated the point of the incarnation.