Kami no Daikousha (Jishou): Subete wa Ano Okata no Oboshimesu Mama ni...... to Iimakuttetara Hiku ni Hikenakunatta

Zero, a young nobleman reincarnated from Japan, finds himself in a world of swords and magic. As a former otaku, his blood starts boiling—he wants to do something chuunibyou-esque! When he discovers his father's embarrassing "Notebook of the World's Truths" in the study, Zero seizes the chance to roleplay. Declaring himself the Emissary of the Holy God, he begins a grand rebellion against the false gods who rule the world—just for fun. But the gods actually exist!? And the girls he casually recruited as subordinates now worship him with complete devotion! Unable to back down, Zero continues to play the part of the Emissary, unknowingly guiding the world from the shadows!

(Source: Kadokawa, translated)