This is the title of and question asked in a 1999 book by two British scholars of mysticism, Timothy Freke and Peter Gandy. It's an excellent summary of the mythological (i.e., non-historical) argument about the origins of Christianity, and a heartbreaking history of the religion as it developed, as well, no matter which side of the debate you're on.
The basic argument is this: Jesus shares an astonishing number of characteristics with Pagan godmen like Osiris, Dionysis, Adonis, Mithra, etc., including virgin birth (on the same day--Dec. 25th or Jan. 6th (ancient winter solstice, depending on the calendar)!); a life spent wandering and performing miracles (changing water into wine at a wedding, raising the dead, healing the sick, feeding the masses with a loaf of bread, catching a miraculous number of fish); and being put to death (on a tree, usually), then rising after three days. Pagans--especially the ones who were initiated into the mysteries around these characters--did not believe that the heroes of the religions were anything but symbols; the stories about them were a means of drawing potential initiates into the mysteries, which would lead them to spiritual enlightenment. Initiates into the Mithra cult (based originally in Persia), for example, would understand that they were on the same mystical journey as those in the Osiris (Egypt) or Dionysis (Greece) cults. The form didn't matter, but the content was universal (among Mediterranean Pagans, anyway).
The Jesus Mysteries hypothesis is that, like Pythagoras who brought the Osiris cult from Egypt (the mother cult of these mystery religions) to Greece to create the Dionysis cult, Alexandrian Jews around 2,000 years ago intended to modernize Judaism by investing it with Pagan ideas, which at that time were the most cutting-edge in the world. These were Jews who read the Torah in Greek (the Septaguint). Not having a pantheon of Gods to pick from to put the godman myth on, the Jewish myth-makers went to the most obvious candidate for godman status, the Messiah figure (Christ, in Greek).
Freke and Gandy lay this out very well in the book, which is written for a nonspecialist audience (but with scrupulous footnotes), asking themselves questions along the way: Can there have been a real person Jesus is based on? What is the evidence of a historical Jesus in the Roman and Jewish record? What did Paul believe?
At the same time, they examine what happened to Christianity itself, how it got separated from its Pagan roots to the extent that it viciously demonized Paganism and turned the gnostic founders of the religion into despised (read: persecuted) heretics. The simple answer to what happened is Literalism, a reading of the scriptural texts as literal history, which is quite a trick considering how strange and inconsistent they are, even internally. Consider that Christianity in Rome was concerned primarily with becoming the state religion, with developing a single way of reading the text and doing the religion, with rooting out heresies and heretical documents, and with "fixing" the record. Consider that virtually all Christians, even today, whether Catholic or Protestant, fundamentalist or mainstream, measure their faith according to how "true" they find the Nicene creed!
As a person who was inclined to subscribe to the mythological argument myself, I found this book totally fascinating. A balm, you might even say, for a skeptical mind. But I would think those of you on the historical side would want to check this book out as well. I'd be suprised if it didn't open some eyes.
Here's a link to the author's home page. Check out their links page, as well.
http://www.jesusmysteries.demon.co.uk/home.html