Much like Neal Gaiman and Terry Pratchett's Good Omens, Only Begotten Daughter left me nonplussed. The sense of humor (other than some assorted wordplay) didn't work for me, and what could have been a wry and subtle story about a divine young woman trying to find her purpose in life takes a horribly wrong turn when the devil himself shows up and proves to be working to use her for his own purposes. Morrow uses the set-up to poke some fun at Christianity, and is sometimes entertaining when he does so, but often the story is muddled.
The first third of the book is best, with child-of-god Julie Katz growing up in New Jersey with her Jewish father, lesbian almost-stepmother, and best friend. Thing go downhill in the middle third, when the adult Julie tries to figure out how to help people, and gets caught up in a web spun by Satan to create a new church. Julie makes some decisions which I just didn't buy about her character, and spends the last third of the book trying to make sense of what her earlier actions created: A fairly standard religious dystopia.
Though Morrow has clearly researched his source material deeply, he has trouble getting to the heart of his characters (Julie is, at best, something of a cipher), and his story isn't particularly effective. The strange "moral" of the story seems to be: If people are chastising you for not reaching your full potential, then lower your potential. Morrow doesn't seem to grasp the irony of this lesson, and the book ends up feeling profoundly unfulfilling.