Making a true believer is a side mission in Ōkami, taking place in Kamiki Village and Sei'an City's Commoner's Quarter. Camille does not believe in the gods, thus prompting Amaterasu to prove herself.
Walkthrough[]
You can find Camille and her sister Camellia at the base of the Guardian Sapling in Kamiki Village, after the mission "Sacred Tree reborn". Do not go through the torii gate, as that will take you into the spirit world. Talk to Camille, then tackle her to receive ten Praise, with dialogue changing depending on when you talk to her. They will remain at Kamiki Village if you do not tackle Camille, but will leave after the festival if you have.
You can run into them again later in Sei'an City's Commoner's Quarter, on the south-west side, across from the bridge. Talk to them again until you are prompted to grow a tree, which you can do by adding a dot on the ground. You will then be prompted to make it rain after growing a tree out of nowhere, which you can only do with the Deluge brush technique. Deluge is only available after "Lone Wolf vs Evil", and requires you to climb Himiko's palace, and go around the entrance to the interior, then talk to the handmaiden.
Follow her request by stepping onto the platform, where you can use a small pool of water with Waterspout to drench all four stones in the distance, causing it to rain. Nuregami will appear afterwards to grant you Deluge, with which you can drench enemies and summon water if there is none to use nearby. The nearby treasure chest with the Waterspout Tech 2 scroll will explain how to use it (by drawing two vertical lines).
Return to Camille and Camellia afterwards, then cause a tree to grow, and immediately follow it up with Deluge, then talk to them again to receive fifty Praise and Stray Bead no. 50