First he discards the virgin birth (let alone the Catholic addition of the Immaculate Conception), suggesting instead that Jesus was the product of a secret marriage between Mary, a temple Ward, (whose own parentage owed more to ancient pagan sexual rites, than to Jewish custom) and a son of Herod, and therefore had the earthly right to be called the King of the Jews. But the historian also makes the argument that Jesus was the Messiah based on many writings of the Jewish prophets. Interestingly, while he dismisses the virgin birth as too mystical, comparing it to Greek and Roman mythology--making Jesus no more than Perseus, fathered by Zeus in the form of a shower of gold (I don't think there's a 21st century meaning there)--he leaves the central miracle of Jesus' life--documenting it, without endorsing it or dismissing it--the resurrection, but more about that later.
What the book primarily does, through Graves' startling knowledge of Greek, Roman and Hebrew history and texts, is to expose how many of those rites and mysteries that the Christians and Jews celebrate have their roots in much older Pagan religions. Most of us know that Christmas was to supplant the celebration of the Winter Solstice, and Easter is drawn from various European traditions of Beltaine, Walpurgis and Eostre, but Graves digs deeper, comparing the rite of communion with the symbolic digestion of the organs in Egyptian mythology among others. The mythology of many religions and cultures feature the sacrifice of a Son or the resurrection of a son into a father.
But what I find most fascinating is that Graves, an atheist, does not dismiss the resurrection, and while he finds practical explanations for Jesus' miracles, the final and ultimate miracle--the point of Christianity itself, he leaves alone.
But perhaps the most interesting/disturbing thing is that his Jesus has come to defeat the feminine in all her guises--not just the ancient cults such as Lilith, but all feminine--only by defeating the feminine can the kingdom of heaven be born on earth. That it was not the apple of knowledge that doomed Adam and Eve, but Eve's insistence on sex and motherhood. He urges his disciples to live with their wives as brother and sister. He takes a bride--Mary, sister of Martha and Lazarus--but despite her wishes, he will not consummate the relationship. They are the holy king and queen, but he has not been consumed by her--she is subjugated to him.
Mary Magdalene is seen as a combination of all the derogatory version of her through history--she is not just a prostitute but a Madam, a witch and possessed by demons that Jesus drives out. There is a long passage describing a debate between Jesus and Mary where she describes the Pagan and he re-translates it into Jewish prophesy, eventually bringing her to his side.
Rather than being an overthrower of Jewish law and tradition, Jesus is described as being the surest defender of the law in the tradition of Rabbi Hillel. This too is a contrast with the "Hippie" Jesus who comes to break with all that has gone before.
It is important to note that this book was written in 1946 and that Graves had something of an obsession with the early feminine cults:
It is not, to my mind, nearly Graves' best book, lacking the humor and realism of the two Claudian novels or the self-depreciating wit of his autobiographical "Good-bye to All That." Still it shakes up the traditional notions and encourages questioning.