Yes, the Omnians. They're a scathing critique of dogmatic hierarchical religions in general and Christianity or maybe more specifically Catholicism and the Spanish inquisition in particular. The zenith of their folly is depicted in Small Gods, where it's revealed that not a single one among them except for one lowly initiate actually believes in their god anymore, having replaced him instead with blind obedience to their rituals and the bureaucracy of the church itself. The Discworld being what it is, their god is very much real but has been diminished to inhabiting the body of an ordinary tortoise and is unable to communicate with anyone except his sole remaining believer.

One of their ironclad declarations is that the Disc is spherical, a notion over which they are quite willing to torture people to death for questioning. Of course as we all know, the truth is the opposite. And yet, the turtle moves.

Much like how various strains of Christians now wear a cross, a literal execution device, the climax of the story explains why "modern" Omnians on the Disc are seen in the other books wearing little turtle pendants: Things culminate in the church leadership (or rather mostly their fantastically corrupt inquisition leader Vorbis) attempting to execute the only actual true believer of Om on a firebox shaped like a massive bronze turtle that's a mockery of Great A'Tuin, the world turtle.

You’re not tricking me… it’s invisible turtles all the way down

It’s swimming in space ocean.

More importantly, is it male or female?

More turtles