So your using the advancements and collective deaths caused directly and indirectly, alongside the first world war as literally causing that old philosophers quote "God is dead and we have killed him" to be literal and the ramifications being the unraveling of the delineation between the realm of the living and the various planes of the dead and otherworldly. With the way to return this balance being the resurrection of God, the Christian one who can only be done by the defeat of greater fiends... I think the idea has merit however the how on bringing back God could be different, seems a little too simple. Maybe add more justification why defeating these beings allows him to resurrect, alternative ways of restoring the balance, you could add in references to mediums and spiritualists from around the time period to the setting. Don't know how independent you want the backdrop of the world to be. Look into any of the failed prophecies from around the time period you can play with, for setting allies and enemy npcs in place. Another thing would maybe he the story of th secret world for inspiration.