Okay so, the premise, a great and terrible god named Om gets reduced to one small irritable tortoise because he’s down to exactly one true believer, a novice monk named Brutha who’s not especially bright but has a perfect memory, and that setup alone would carry a lesser book. Pratchett does more with it. The actual target isn’t god, it’s the church that grew up around him, the Quisition and its sociopath enforcer Vorbis, the whole machinery of institutional belief that’s replaced the believing itself. That distinction, belief in the god versus belief in the organization built on top of it, is the thing that stuck with me longest, way past the jokes.
And there are a lot of jokes, don’t let the theology fool you into thinking this is dour. Om spends half the book being a sarcastic, put-upon turtle who can’t get anyone to listen to him except the one guy dumb enough or honest enough to keep believing, and the dialogue between them carries entire chapters on wit alone. I read one review calling this the darkest and edgiest Pratchett up to that point and I don’t disagree, there’s real cruelty in here, Vorbis is genuinely unsettling in a way comic fantasy doesn’t usually risk.
Where I’ll push back on the consensus a little: I don’t think it needs you to have read any other Discworld book first, it’s built to stand alone and it does. Someone said they’d have enjoyed it more as a teenager and maybe, but I read it for the first time last year and it landed just fine on an adult who mostly reads about tortoises to relax after work. The ending is the part I keep coming back to though, quietly devastating in a way the jokes don’t prepare you for.