'Just because you can't explain it, doesn't mean it's a miracle.'
 
In the beginning was the Word. And the Word was: 'Hey, you!' This is the Discworld, after all, and religion is a controversial business. 
 
Everyone has their own opinion, and indeed their own gods, of every shape and size, and all elbowing for space at the top. In such a competitive environment, shape and size can be pretty crucial to make one's presence felt. 
 
So it's certainly not helpful to be reduced to appearing in the form of a tortoise, a manifestation far below god-like status in anyone's book.
 
In such instances, you need an acolyte, and fast: for the Great God Om, Brutha the novice is the Chosen One - or at least the only One available. He wants peace and justice and brotherly love. He also wants the Inquisition to stop torturing him now, please . . .