After having written some million words covering more than one hundred RQ cults, I can confidently say that I find all of them fascinating in their own way. Each represents an archetype present in the world, is the manifestation of the divine in the mundane world. This is as true for the Greater Gods such as Orlanth, Ernalda, Yelm, Red Goddess, etc., as for such entities as Yinkin, Donandar, or Yelorna.
Gloranthans have little concern regarding theodicy – the gods are not all-powerful, all-wise, or all-good. They are worshiped because they made the world we mortals live in, and that their cults provide a means for mortal survival within Time.
As an aside, it is worth contemplating that the Cults books represent a significantly greater word count than the Guide to Glorantha itself. Between half again and twice as much (depends on whether I count the stuff I wrote but will publish outside of Cults). It just is a HUGE amount of material.
What happens to non-believers (atheists like the Brithini) when they die? What do you mean by a “non-believer”? What happens to the dead is that their spirit is severed from their body, which will decompose or whatever. Now what that means depends on your spiritual understanding. But it is widely known that the spirits of the dead go to the Underworld and to the Court or Place of the Dead (whatever it may be called).
A few extreme materialists hold that the self requires the spirit+living body to be “you”. So when the spirit and the body are separated, the spirit is mere energy (life force) and the body mere raw materials. The Brithini are the radical materialists of whom I speak. They view the “self” as the ego combination of spirit and body. When those two are separated there is no “I” left, just energy and matter.
Initiation into a cult often provides a guarantee as to one’s fate after death. Orlanth will bring the souls of his initiates to his hall regardless of how or where they die. From there they feast and enjoy, until it is time to return to the mundane world in a new form.