This is a sequel to the novella Black Easter, and to me was an anti-climax. What Stoker's Dracula and King's Salem's Lot is to vampires, Black Easter is to demons. Blish said in his Author's Note to that first novel that every one of the "novels, poems and plays about magic and witchcraft" he's read treat it as "romantic or playful." He sought to write a treatment that "neither romanticizes magic nor treats it as a game." That book is dedicated to C.S. Lewis and even included an extensive quotation from his Screwtape Letters heading one of the chapters. So although I'm not sure I'd classify this as out and out Christian fiction, this does come out of that world view and takes the demonic seriously--that's what does make it unusual and at times fascinating. It's obvious not just from his note but the vividness of his details and even the quotes heading chapters Blish did extensive research--actually reading grimoires and manuscripts on ceremonial magic. At the same time Blish is best known as a science fiction writer, and approaches magic with almost scientific rigor. This sequel though is not so much Christian allegory of scientific hubris as Cold War parody--and I found that more dated and less interesting, although it did have moments of (very black) comedy. And maybe a believing Christian would have found the ending powerful and moving, as the ending of the first book was intended to be shocking--I found it trite.