This book is about how, after JC's death, there was a struggle between heretic believers- those who believed in personal enlightenment and shunned a church hierarchy- and the disciple of John's beievers- patriarchal, hierarchal, congregational- and why John's side won. GG is based on the gnostic gospels which were discovered in urns buried in a cave in Egypt 1950. They had likely been suppressed by the dominant faction.

It does a good job describing how present-day Christianity evolved and, in a sense, how many other religions evolve, for good or for ill. The pattern is as such: leader dies; followers dispute what his teachings meant and diverge into different paths; one branch gains dominance and spends a lot of it's time exterminating the others. This is the same in Islam, probably also in Judaeism, etc.
cg