decay of religion
Cultures are always changing. The rare unchanging cultures become stagnant and eventually die. The past cultures which some people idealize were never anywhere close to “pure” or “perfect” - however one conceptualizes that supposed purity/perfection - and even if they were, most people still would not want to live in them because they’d only be well-suited for a small portion of the population. (Just like a lot of actual cultures, which I suppose is “fine” if you want a semi-dystopia where you are among the people on top just so you can feel superior. Unless you’re delusional about ending up on top even in your own idealized world. Which is not actually fine for most other people, either way.)
Religion gets constantly reinterpreted in the context of the current culture. Short of isolated cults or religious enclaves, you can’t force religion to remain “uncorrupted” for long periods - and unless the religion is relatively young, you’re just going to be preserving a particular “corrupted” form of it anyway. Also I specified “long periods” because cults and enclaves only last somewhere from a few decades to a few centuries (or sometimes even only a few years to a decade in the case of personality-based cults) before becoming more mainstream or dying out.
“Genetic decay” is not really a thing, unless you’re talking about lack of genetic diversity due to limited, closed breeding pools, which dooms a species to extinction if the environment changes too fast. Or, I suppose, it could be true in a metaphorical sense if the world was full of radioactive mutants or something. I assume that this is a dogwhistle for race-mixing.