Between 600 and 1650, the Popes were the leading authority figures of western Europe. How and why did the popes emerge as the heads of the Catholic church?
Early Christianity was a highly diverse religion with multiple centres of power. It was not until it was established as a state religion of the Roman Empire by the Emperor Constantine, that it began to attempt to consolidate itself into a uniform hierarchy. There were two major centres of power at this time, Rome, the capitol of the Latin West, and Constantinople, the seat of Constantine and centre of the eastern, or Greek part of the Roman empire. Thus the Patriarch of Constantinople and the Bishop of Rome were de facto the most power figures in the church.
As the western Roman Empire collapsed into a group of small warring kingdoms, the only centralized authority left which had a network of subordinates across all of western Europe was the Papacy, and thus it filled the power vacuum. Thus as secular kingdoms began to grow stronger in the late middle ages, the came into conflict with the Papacy.