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, they shouldn't be taken at face value. The whole idea that antiquity was full of stories of dying and resurrected gods originated with Sir James Frazer's book The Golden Bough, which dates from the early years of the 20th century and is no longer taken seriously by most social anthropologists.
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.
Their argument seems to me to be extremely far-fetched, at least in the 'strong' form that they advance it. It might become slightly 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 unpersuasive. But the writing is not as bad as some of the rather lurid publicity about it might suggest. It provides references for its claims although curiously the (singularly unappealing) translation of the New Testament which is cited throughout is not specified.