This is the story of Jesus in Jesus's own words. The Gospel According to the Son is a re-creation of the world through which Jesus walked. Ultimately, Mailer convinces, more than any writer before him, that for Jesus the man it could have been just like this.
If Jesus were write his autobography -- as a simple man talking to real peaople, this is what I'd imagine he'd say. The parts where he talks to his Father, talks to the Devil, and is frustrated by his disciples are truly insightful.