The slow changes to the Gefendur religion are actually really neat, I have to say. Bell's weird "tasting the blood of the slain so as to consume their essence and bring their strength to that of his cause" is a good example of this because it's a factual part of the Gefendur religion that just... went away. I have to imagine that as Gefendurism became (outside of Alderode) more centralized under the Ulestrian Theo-Matriarchy and the Crescian Monarchy that such old customs were slowly eroded, and the rest of the world took note. I can't see a well-oiled machine like Cresce, sporting a centrally-planned economy and the abolishment of greed, tolerating a practice as messy as regular human sacrifice.
The fact that Ssaelism or the Paternal-Gefendur heresy even happened is a testament to that. After humans beat back the Senet beasts, the whole "reason" for Gefendurism to even be a thing just kind of faded away. If Gefendurism still needed to be exactly what it was in ancient times, Ssael would have died a quack and no one would have even thought to try and demote Yerta, even a bunch of weirdos up north.
What I find interesting about Gefendurism is the (bizarre) comparison to Raw Head which is a very different kind of gay than Cope seems to be into
. Gefendur exults the female, doesn't really have any big taboos on sex or sexuality, and came about as almost an excuse for humans to go around and murder the Senet Beasts. Before Pymary became popular (and well understood), the one real advantage humans had over Senet Beasts would have been that human can reproduce and the Senet beasts can't. The prospect of your Slave-Worshipers growing in numbers to the point where they can just mob you to death is one that only an unaging immortal that can't breed has to worry about.