This is the thirteenth Discworld book, a stand-alone story set in Omnia. It tells the story of Brutha, a trainee monk who has a photographic memory and the Great God Om, who is currently stuck in the form of a tortoise.
The Omnian religion is one of fire and brimstone. It has a Quisition that kidnaps people and tortures them for believing in gods other than Om, or for believing that the world is flat. Both of these things are true in the Discworld.
The Quisition is run by a man named Vorbis, who is utterly ruthless. He is planning a diplomatic journey to the country of Ephebe, and decides to bring Brutha along as he thinks his memory will be useful. Of course, Vorbis is scheming and lying, and he plans to overthrow Ephebe.
During this time Brutha is communicating with the God Om, and he is the only one who can hear him. Brutha at first suspects that the voice is a demon, and Om’s reactions to this are very funny. Om has lost his power and is determined to get it back. It becomes clear that the reason he is a tortoise, and the reason that only Brutha can hear him is because he is the only person left who truly believes. Everyone else believes in the church, or the rituals, or simply fears the Quisition.
The book opens with a parable about eagles and tortoises. Apparently eagles eat tortoises by picking them up and dropping them onto rocks. It discusses why eagles do this when they have plenty of other available food sources, as well as what this experience must be like for the tortoise. It must be like a religious experience where their horizons are broadened to an extent they were not aware was even possible. They are unaware that they are being murdered by the eagle, let alone that it is probably doing it for amusement. Om has this literally happen to him just before the opening of the book, and we find out that this is how he came to be where Brutha found him. More than this though, it is also what happens to Brutha in the story. Vorbis is the eagle, and the trip to Ephebe is the broadened horizons. Vorbis treats people like playthings, he has no care for the suffering of others, and he enjoys it.
Clearly Pratchett is using a metaphor here for real-world religions and faith, but more widely human nature. It is a very humanist book about how people’s actions are what matter. Rituals, clothing, sermons, and prayers are worth nothing in comparison to the person. I grew up reading Discworld books, and I wonder how much they shaped me and my views or whether I was drawn to them because of the core of humanity in them.