Famuli vestrae pietatis

Famuli vestrae pietatis, also known by the Latin mnemonic duo sunt ('there are two'), is a letter written in 494 by Pope Gelasius I to Byzantine Emperor Anastasius I Dicorus on the relationship between religious and secular officials.

Description
Famuli vestrae pietatis is a letter written in 494 by Pope Gelasius I to Byzantine Emperor Anastasius I Dicorus which expressed the Gelasian doctrine. According to commentary in the Enchiridion symbolorum, the letter is "the most celebrated document of the ancient Church concerning the two powers on earth." The Gelasian doctrine articulates a Christian theology about division of authority and power. All Medieval theories about division of power between priestly spiritual authority and secular temporal authority were versions of the Gelasian doctrine.

According to the Gelasian doctrine, secular temporal authority is inferior to priestly spiritual authority since a priestly spiritual authority is responsible for the eternal condition of both a secular temporal authority and the subjects of that secular temporal authority but "implies that the priestly authority is inferior to the secular authority in the secular domain."

Dualistic principle of Church and State
This letter established the dualistic principle that would underlie all Western European political thought for almost a millennium. Gelasius expressed a distinction between two principles governing the world, which Gelasius called the "sacred authority of bishops" (auctoritas sacrata pontificum) and the "royal power" (regalis potestas).

Potestas and auctoritas
These two principles—auctoritas lending justification to potestas, and potestas providing the executive strength for auctoritas—were, Gelasius said, to be considered independent in their own spheres of operation, yet expected to work together in harmony.