There have been quite a few attempts in the last hundred years to cast doubt on the historical existence of Jesus, but this book goes further than most in seeking to make a link between the Scriptural account of Jesus and the Mystery cults of antiquity. In the early years of Christianity there were numerous Gnostic sects claiming esoteric knowledge. These sects were subsequently declared heretical by mainstream Christians but the authors' thesis is that Christianity was Gnostic at its origin. They suggest that at some time before the Christian era a group of Jews produced a Jewish version of a pagan Mystery cult, based on the Messiah, with a fictional Jesus as a dying and resurrecting godman resembling Osiris-Dionysus. In time, this esoteric teaching came to be interpreted as historical fact and what they term Literalist Christianity was the result.
It has of course often been remarked that the theme of the dying and resurrected god is not original to Christianity but was widely found in Near Eastern religions. But the book cites more parallels than this. claiming that Dionysus is hailed as 'The Saviour of Mankind' and 'The Son of God'; his father is God and his mother is a mortal virgin who is later worshipped as the 'Mother of God'; he is born in a cowshed; he drives out demons, turns water into wine, and raises people from the dead; and he rides triumphantly into town while people wave palm fronds to welcome him. The first Christians revered Dionysus's birthday as Jesus's birthday and the three-day Spring festival of Dionysus, celebrating his death and resurrection, coincides with Easter. There are resemblances between Dionysian rites and the Last Supper and Eucharist.
Striking though these parallels appear, I'm not sure that they should be taken at face value. In any case, the major difficulty with the authors' theory is its sheer improbability. Would the Jews of this era have adopted such pagan ideas? Freke and Gandy acknowledge this problem but claim that there were many points of contact between Jews, especially diaspora Jews, and contemporary pagans. But this evidence is all indirect; the only direct evidence for their theory is the Jesus story itself, and that cannot be adduced in support of the thesis without falling into circularity.
I have to say that their argument seems to me to be pretty far-fetched, at least in the 'strong' form that they advance it. It might become more plausible in a 'weaker' form, if we postulate that there was a historical figure called Jesus on whose life story a number of elements from the Mystery religions were later grafted, although I find even this weaker form to be fairly unpersuasive. But the book is better than some of the rather lurid publicity about it might suggest. It contains some surprising information and is well referenced, although curiously the (singularly unappealing) translation of the New Testament which is cited throughout is not specified.
For a good discussion of this question see Did Jesus Exist?, by Bart D. Ehrman.