Church of Leto is a religion based in Reole, Eastern Amestris and is an organization in Fullmetal Alchemist. It was founded by Father Cornello after he had claimed himself to be the Messanger of God and performed many miracles in order to prove it.
He did this by using the Philosopher Stone (Red Stone in the 2003 Anime Adaptation) and lied and manipulated the townspeople into worshipping him and the sun god Leto. He would then lead the followers of the church into joining his cause by becoming an army of fanatical zealots whose goal was to overthrow the Amestrian Government and conquere all of Amestris.
The Church had existed for an unknown period of time before the arrival of Edward and Alphonse Elric in which the two of them exposed Father Cornello for the false prophet that he truely was and fought and beaten him. In the end, Cornello was overthrown and devoured by Gluttony once he is caught failing. In Fullmetal Alchemist: Brotherhood, Father Cornello is posed by Envy who continues to perform false miracles and convinces the town people that he is the real Father Cornello and leads the church once more.