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From my understanding Diana worshiped and prayd to the current Godess who is the center of the curent religion.
However it has been stated and show with the statue in the temple of their Godess killing / striking down another Godess, who is mentioned as "the First".
So the worlds original Godess was betrayed by a lesser Godess who took her place. The First Godess is most likely the "Hestia" that Hestia has meet in her dreams that has guided her and the reason Hestia was sent back in time/ has her memories in tact.
The current Godess has been playing the world like a novel and massively favores Diana (shaping the world in her image).
Hestia having the blessing/ may even be the vessel of the First Godess, making her immune to Diana and the current Goddeses interference.
This is slowly restoring the world balance to its original state, making Diana's godess lose her power over the world = Diana's true self becomming more prominent. I think this is why she is nothing like a saint anymore.
Diana has slowly started to hate Hestia because her world that has always bended to her vim and will, now diverged and changed. Normaly no matter how badly Diana betrayed someone, they would still love her. But with Hestia here, ppl are turning away because there is more than one road to take.
My guess is that the bigger the impact Hestia does on "the story", the more power is restored to The First Godess. This is slowly freeing the ppl of the world from "the story's narative" which was mostlikely made by the current Godess to elevate Diana whom the current godess favores.
Diana losses her power as she becomes more selfish and neglects her praising of the godess, meaning her power and the godess are intertwined.
The current godess most likely "resets" the world after Diana losses her powers as the more Diana is praised and worshiped, the more powerfull the current godess is.