Since olden times, it has been ritual for a ceremony to be held to choose the priests of the seals. A priest of each nation is chosen, forming an organization of clergymen which crosses the boundaries of religious sects. Standing at the head of this organization is the hierarch.
As the sole living being allowed to make contact with the goddess, his most important duty is maintaining her safety as the final holy seal. The hierarch is the only one able to apply the seal to a goddess as well. He serves as the supreme commander of the Union (and later, the Knights of the Seal); his political power is tremendous. However, the hierarch is not attached to any one religion or organization, serving to the bitter end his official duty as "The priest who exists to control the seals," so long as he stands in a position to oversee it.