There's no ambiguity here. You aren't left wondering whether it's a fantasy or just a fantastic premise. The reader learns pretty much from the start that there are renegade angels who fathered children by human women. In modern times there are still descendents of those angels.
There's a book and there's an autistic kid who seems to be the only one who can read the book. There's a priest who solves problems for a cardinal at the Vatican. He seems to be at odds with other henchmen of the same cardinal at the Vatican. It's somewhat confusing.
What is ultimately confusing is why the angels must destroy their descendents in order to get back in God's good graces. Why would killing people be what God wants? Also the mystic book that supposedly tells God's secrets to humans, if only they could read it, contains pictures of plants and chubby naked women. If God's secrets involve plants and chubby naked women, perhaps we don't really need to know them.