This will be another pretty short review, to avoid spoiling the last book, The Man from Pomegranate Street, as some of the story here sets up the conclusion to that book. There are minor spoilers for The Prophet from Ephesus here, though.
These books are set during the first century AD, when Christianity was a very new religion, and this volume is about the rise of Christianity, taking place in modern Turkey, where our heroes are able to meet Jesus' disciple John while investigating more kidnappings of young children. Earlier books in the series had referred to Christianity as well, of course, but this is the only one to focus on it almost exclusively. Over the course of the story, Jonathan rediscovers his faith and Nubia and Lupus, along with several old acquaintances, convert as well. Several different aspects of early Christianity (of which we know very little for sure) are depicted; the more contemplative side is represented by John, but we also see the hysteria and intensity of the crowd in some early scenes. Most heartbreakingly, we see Lupus caught up in claims of miraculous healings, which can only end in disappointment for him.
The book does not, however, fall into the CS Lewis trap of presenting only one point of view. Flavia does not convert, and her isolation when her friends get swept up in a baffling new cult that she cannot understand is just as heartbreaking. Looking for her father and frustrated in her renewed desire to get married, her loneliness enhances how alien and frightening this strange religion is to her. This provides a vital balance to the enthusiasm of the others.
Perhaps appropriately, there are not many answers at the end of this book. The mystery is only half-solved, Flavia and Nubia's burgeoning relationships are dangling in the air, and the power of the new religion to effect real change in people is suggested but as yet unproven. This seems particularly appropriate for the penultimate volume in the series, as the children grow further towards the adult world in all its murky greys. Overall, the story is a touching exploration of how some people may have encountered Christianity back when it was a relatively obscure cult surrounded by strange claims and in a constant state of flux as it developed, leaving no one quite sure where they are going but perhaps a little more confident of getting there.