Freke and Gandy believe that first century Jewish mystics adapted the potent symbolism of the Osiris - Dionysus myths into a myth of their own, the hero of which was the Jewish dying and resurrecting godman Jesus. Therefore, the story of Jesus is a consciously crafted vehicle for encoded spiritual teachings created by Jewish Gnostics.
We are unaware of this, they claim, because the Roman Catholic Church destroyed evidence of the connection between Christianity and the pagan mysteries. They make their case by offering an examination of mystery religions, especially Greek, pointing out the many parallels between them and what they see as the Gospels! message about Jesus.
Freke and Gandy are familiar with a significant amount of recent biblical scholarship, though they rely mostly on Elaine Pagels's work on the Gnostics.