Wow a Japanese story where the Gods are evil and the demons were the good guys. How completely original and unexpected. I'm sure the church being corrupt is something nobody will ever see coming either.
This isnt a possession. This is him remembering his past life late and his outlook and personality being affected by that. I can understand why you are confused since he keeps referring to his past in third person as if that wasnt him.
I don't understand why the MC is treating his past like it didn't happen to him. This isn't a transmigration or possession, this is reincarnation where he just remembered his past life late but he keeps mentioning his past in third person as if it didn't involve him. It's not as if he lived that...
And now i remember why I stopped reading the novel version. That collar gag wasn't funny the first time but the author beats the joke to death by using it all the time and it gets annoying FAST.