Vorbis, a religious man in his own mind, gets tripped up by his own mind - and a man, and a turtle. Pratchett takes on organised religion, essentially, and it's funny and horrifying all at once. For a lot of people I know this is their favourite Pratchett, but Brutha has never had the same appeal to me as Granny or Vimes, for instance. I don't know that he's cynical enough, though I suppose Om has cynicism for the both of them.