@Academia Nut, what "Universal Truth" is the prophet preaching of? What "sophisticated but false" teachings is he claiming the priests teach?
Here:
Universality
It basically is a push against relative and circumstantial facts and a push towards objective universally true things. The drawback is that well...it doesn't recognize mitigating circumstances and realpolitik.
It's philosophically important in the development of formal logic, though applying it everywhere in life goes about as well as you might expect. It's scientifically important in the development of elaborate measurement techniques, though lacking these tools will be agonizing and produce all kinds of hilarity.
Socially it potentially means a push towards these troublesome events:
-Equal punishments for the same crimes. Your social status doesn't matter, nor does the status of the victim but neither does self defense or coercion.
-Judgementalism of foreign parties intensifies. Expect pushes against foreign slavery to show up again, as well as environmentalism, etc. It's all about imposing your standards on others.
-Scientific/experimental rigor. Truth is universal, therefore finding out why similar things produce different outcomes becomes very important.
-Economical universalism...well, bring the popcorn. Attempting to derive a unified value of any commodity or trade good is liable to be a frustrating and very very expensive endeavor.
-People trying to extend their personal experiences as universal truths. Cue offensive stereotypes proliferating.
Teething pains basically.
It's a good and useful philosophy, it just needs some maturation first to develop essential nuances.
Ideally, we have him debate this in public, have his arguments torn apart and then rebuilt stronger than ever.
Narratively, it's a consequence of our choice to Embrace the Absurd(which produced philosophical concepts that are kind of too early for our tech level
). The universe being contradictory, nothing being certain, the deaths being a terrible loss yet considered a virtue and honor...this is the pushback.
People want some certainty back in their life, something to believe in. Universal cardinal sins to blame for all ills.