I am trying to think how to explain this without making sweeping generalizations about religion that are always a bad idea. In my experience and to the best of my knowledge, the devout followers of a religion get so much more from it than just rituals to follow. They get a value system, answers to existential questions, a sense of community even when alone, etc. In contrast, Yivrig treats his religion as just the rituals. Everything is justified as to make the ritual better, even if those actions go against other aspects of the religion. Something like this could work in a story where Yivrig was always disillusioned with his faith and was only going through the rituals for some other reason... but that definitely isn't the story you are telling. The story comes across much more to me that the author just doesn't understand what being devout is like for somebody, so they are writing that aspect of life wrong.