On this page of the Assassin's Creed Valhalla Siege of Paris guide, you will find a detailed walkthrough of the Not God Enough side quest.
Starting the mission
This mission will begin in the western part of the region, south of the synchronization point.
You will hear voices from the church. Break out one of the stained glass windows and go inside. Defeat all 4 opponents so that no one will attack you later.
After killing the enemies, you must approach the statue located in the middle of the building. There is a note lying under it - read it to complete the task.