A belief that built upon Jewish belief, but mutated it in unique ways? A single person resurrected in history, a physical but transformed body, a division between the resurrection of the messiah and then the final resurrection etc?
The empty tomb and the appearances of Jesus are obviously a good explanation.
Any other explanation - dreams, visions, religious hysteria, cultish deceit - must still account for the emergence of the unique Christian beliefs themselves. Often sceptical accounts of the rise of Christianity don't reckon enough with how remarkable and unique the Christian belief system is:
The real problem is something that any first-century historian should recognize: that whatever it was that the early Christians were expecting, wanting, hoping, and praying for, this was not what they said, after Easter, had happened. (p. 699)
So argues N. T. Wright in The Resurrection of the Son of God, Chapter 18 "Easter and History"
