It is commonly believed that the Bishop of Rome (the Pope) held supreme authority over the Church during the Middle Ages and that his power was broken only by the Protestant Reformation.
In fact, the papacy fluctuated in power quite a bit during the Middle Ages and the office was not acknowledged as supreme until after the Protestant Reformation.
The Bishop of Rome was the only Western patriarchate in the Roman Empire and when the Western half of the Empire disintegrated in the 5th century CE, it was relegated to something of a frontier status. After the fall, the other three patriarchs actually elevated the bishop of Constantinople to patriarchate status precisely because Rome was no longer part of the Empire and the power had shifted east. Of course, the bishop of Rome protested this move and never acknowledged it. The legates literally packed up their toys and went home.
Rome became prominent as a secular power long before it was a powerful religious force in Europe. The original interactions with Pepin the Short and Charlemagne – the Carolingian kings of the Franks – came about because the bishop of Rome held lands in Italy that were threatened by the Lombards. Both kings helped Rome out in return for favors – culminating in the pope crowning Charlemagne as “Emperor of the Romans” in 800 CE.
But the relationship between Rome and the rest of Europe was not necessarily a peaceful one. During the 11th century CE, there was a conflict between Rome and the Germans known as the “Investiture Controversy” during which the German emperors simply appointed competing popes and refused to acknowledge the pope. Then came the Avignon Papacy (1309-1378), during which the popes were all French and ruled from a French resort town. And there was the Papal Schism (1379-1417) when there were sometimes as many as three popes all claiming apostolic authority.
It was only in the early 15th century that the pope consolidated his power and relocated to Rome. In 1450, Nicholas V declared a Jubilee in Rome. Throughout the year, pilgrims flocked to the city to gain indulgences and to fill the coffers of the churches there.
After Nicholas V, there was a rapid succession of powerful popes drawn from the most influential families of Italy who used their power and money to leverage themselves over most of Europe. It was this corrupt papacy, filled with Medicis and Borgias, that Martin Luther rebelled against – a papacy which really had only held prominence for less than a century.
There’s a lot more stuff that occurred during the Middle Ages, but hopefully this illustrates the true nature of the papacy during the Middle Ages. I haven’t even touched on the views of the Patriarchs in the east who represented the vast majority of Christians until the coming of the Muslims in the 7th century CE.