The almighty Tree God was partying with Jesus one day (Jesus followed the Tree God).
Jesus had some crazy idea while he was drunk, he said "eh Tree God, I was thinking about making my own religion, but i have nothing to convey my message with". The tree god said: My sweet little Jesus, have some thin sheets of my ass so you can write your holy words and share them with the world".

Then the Tree God gave Jesus paper, and Jesus wrote the bible. Now everytime anyone reads the bible, they are sworn in to Treegodism.