This could actually be a great idea. Dunno about the theological part, it's just a plot device but could be good too.
It kinda reminds me of Legacy of Kain, where there's an Elder God that feeds on souls and requires the wheel of fate to keep turning.
Vampires were an interesting boon due to the amazing amounts of death they caused but in time, they almost exterminated the whole world and every vampire is immortal so their souls are beyond his reach, so he sets Raziel to go after them to end their immortal life.
If I were to take this premise, you could instead say that the world has a limited static number of souls that are reborn over and over.
The Elder God feeds on the emotions of the living / the knowledge they had upon death / their life experiences / anything, really. But since he exists beyond the mortal realm, he can only feed on souls when they die.
He can't manifest himself but he can influence the world in several ways, creating war, famine, plagues and strife everywhere to keep people dying all the time. Natural death is actually a curse placed upon their souls before reincarnating.
Due to the constant death and war, no civilization advances too much. They can't travel to another plane of existance (thus they will never confront this Elder God) and their religious practices (created by a (((prophet))) of this Elder God) keep them indocrinated into believeing in an afterlife.
The MC would find some ancient ritual or a method that accidentally removes his soul curse and thus his body no longer deteriorates.
Instead, he starts to rise in power and knowledge, understanding more and more of the world and seeing everyone else as the lesser souls they are and what they could be, so he makes it his quest to free them from their curse as well.
The ultimate goal is for everyone to join the MC in the ranks of an immortal race of living souls that never ages and only accumulates knowledge. The Elder God fears this as not only it kills his source of food, it also means humans will eventually find a way to travel into his realm and fuck him up.
The MC's powers are indeed magic in a world where magic is very, very light for most people. However, as his knowledge of the world increases from not dying at all, his powers grow to incredibly ridiculous amounts to the point that the Elder God has to send several horrible abominations and finnaly trying to cause Armageddon just to stop you and kill everyone so nobody remenbers any of this.
It will fail since the MC will prevent it, build an Arkane Ark and take 100 of his best Battlemages with him, travel through a tear into the Elder God realm and fuck his shit up.
Then when the credits roll, you see several other universes, each controlled by their own Elder God and your task is now to fuck all of them up and liberate every other race. The potential for expansions and sequels is limitless.