Divine right of kings
Pharaohs and some other ancient Kings were thought to actually be gods or descendants of gods. That concept went away when monotheism spread. A medieval principle held that God gave the right in to rule in secular matters to political rulers. God gave the right to rule over spiritual matters to the Pope as head of the Church. This has been called the 'doctrine of the two swords'. In 800, Pope Leo III crowned Charlemagne as the Holy Roman Emperor. From that time on the Emperor was the secular power while the Pope was the spiritual power. This concept began to break down prior to the First Crusade. Rather than have the Emperor provide the manpower for the crusade, the Pope called on the nobility of Europe directly.