Reformation is a social policy in Civilization V. It is part of the Piety tree and requires Religious Tolerance. In a mature religious society, it becomes inevitable that a drive to change manifests itself at some point. As religious practices stagnate and become too gaudy and empty of spirit, people start clamoring for a reform. And this Reformation brings about new life in their main Religion - a new Belief which is so strong that may potentially alter the course of your empire!