. Neither of them are believers except in the narrow definitions of their own perceptions of what their religion or philosophy means to them
. No one else. So it is the usual treating people as things concept but in SG it's much more thought through and cogent. The books do not bear comparison in most respects because they're focussed on different aspects, and with Pyramids, this is not
religion or even philosophy, but it is about time and stagnancy and what happens when people are not allowed to change, or to mix with the rest of the world, because their society is so closed in upon itself. There is a measure of that in SG, but there it's outward-facing and with Vorbis in the driving seat will become aggressively expansionist - indeed it's meant to be that way, but Lu-tze saves it by rescuing Om and making sure that Brutha finds him.
Dios, as you say is mostly a passive tyrant and his obsession does not hurt the living as such because he does not seek to dominate his nominal lords outside of the rituals he's devised for his imaginary gods and the culture of life after death. The gods are irrelevant and so aren't
small gods. They don't manifest for thousands of years, because they're not worshipped and have no existence whatsoever, outside of being the raison d'etre
for Dios' pointless rituals and customs. So long as the High Priests and Priestesses toe the line for him in return for the power they wield, and with nothing to smite back at them because the cycle of renewal for Dios is sustained through the powerful magic perpetuated by the pyramids. So really Pyramids has very little to do with faith because nobody actually cares about the 'gods' - and belief such as it is, is all directed to the funeral myths and continuance of their kings lives after death, such as it is.
It's the gods that make the difference between these two priests. No question Vorbis is vile and vicious but, like Dios he moulds his god to fit him, even though Om is very much 'authentic' and does exist. Like Dios Vorbis finds his way to power through manipulating the faith for his own ends and, like Dios, he has no belief beyond what suits him and his aims. They are alike, but follow different paths. Om cannot do anything about Vorbis except through Brutha - the small gods cannot get through his adamantine mind, so he's useless to them. Even though he sees himself as a believer and the deliverer of truth, it's all in terms of the power he can wield and the fear and dominance of everyone that he glories in. He ruthlessly pares everything and everyone down to the core, the bone, and holds it all in contempt in the name of a faith he does not have. He appears to serve but, in the centre of his being he is alone and knows it. When he is hailed as the new Prophet he fears Brutha, yet it is Brutha who is his only friend and is the one who has to take him through the last desert.
Dios too, is alone in his faith, the difference being that of course he's the one who created it and fuelled it with continuity. He's a more sympathetic character than Vorbis in that he's not a torturer, except of himself in most senses. With most of Terry's villains there's a spark of humanity or some quality or saving grace (like Mr. Pin's belief in potatoes or Reacher Gilt's swashbuckling style), that makes them not wholly despicable, but Dios has few admirable qualities to merit sympathy and Vorbis, none at all. We're glad to see him completly frozen, surrounded by the fear of his victim's ghosts. We're pleased he stays there for 100 years of Brutha putting Omnia back onto a healthier path, whilst for him every second lasts 1000 years. Although there's satisfaction in seeing Dios, deposited still living, right back at the beginning of the time cycle, there's a kind of horror to it, because he's achieved his ultimate aim. Dios is a self-created god, immortal and stuck on an eternal wheel of his own creation with nothing to look forward to and no respite from a life that staves off death, but otherwise has no meaning and no end.
Terry's version of death honours belief. A desert to cross seems to be a constant in the majority of the books, but people do get what they're expecting by and large. Dios and Vorbis are the only ones who don't want and truly fear death and so get the 'end' they deserve.