Many people now mistakenly think, "In the Middle Ages, people had to do whatever the pope said." This is far from true. Most of the time, most people didn't even know who the pope was.
Let's start at the beginning. As Christian communities were first established in the late first and second centuries AD, they acquired bishops, leaders who were locally elected (click here for more on bishops). The bishops originally ran Christianity between them, holding conferences to decide important issues. By the fourth and fifth centuries, as Christianity became first tolerated in the Roman Empire and then eventually the official imperial religion, five of these bishops became "first among equals," the five patriarchs as they were known.
The five patriarchs were the bishops of Rome (naturally), of Constantinople (the "new Rome" since the imperial capital was moved there in the fourth century), of Jerusalem (naturally), of Alexandria (in Egypt, the greatest center of learning in the ancient world), and of Antioch. Antioch was where the apostle Peter went, according to the New Testament. In Latin, all patriarchs, and many other bishops, were called papa, meaning "father."
The bishop of Rome, the only patriarch in western Europe, was looked up to by Europeans, and he took over effective governance of the city of Rome once the emperors were off in the East. He also encouraged the Christianization of European territories not already Christian; for example, around the year 600 Gregory I first sent missionaries to the pagan Anglo-Saxons in England. By this time, the bishops of Rome were arguing that the apostle Peter had been their first bishop, having come to Rome from Antioch, joining Paul, whom the New Testament puts in Rome (though not as bishop).
In the seventh century, with the rise of Islam, the Christian communities in Jerusalem, Antioch, and Alexandria essentially disappeared, leaving just the patriarchs of Constantinople and Rome. East and West, Greek-speaking and Latin-speaking, drew further and further apart. From this point on, one can really call the Roman pontiff "the pope."
But popes still had no real authority outside of Rome. People loved to make pilgrimages to Rome, to visit the catacombs under the city where early Christians were buried (and to snag a bone or two to take home as a relic), to visit the Tomb of Peter. But a lot of popes were non-entities, sons of powerful Roman families put into office by their relatives, who had dropped the previous pope into the Tiber. They had constant problems with the Lombard kings (northern Italy) and the Saracens (southern Italy).
The highpoint for early medieval popes was the coronation of Charlemagne in the year 800. Charlemagne was king of "Francia" (essentially France and western Germany), and he had come to Italy at the pope's request to beat up the Lombards. As a reward, on Christian Day 800, in Saint Peter's basilica, the pope crowned him Roman Emperor.
The pope was convinced at the time that the Roman Emperor in Constantinople was a heretic, following a deviant version of Christianity. To make it worse, the emperor of 800 was a woman. From now on, there were two Roman Emperors, one in Constantinople and one in France or Germany.
The western one (not the eastern one) had the precedent of having to be crowned by the pope to be considered truly the emperor. Many a German king in the next two centuries came to Italy to be crowned emperor, even if he first had to beat up the pope to get him to do it. But the popes still had little authority the rest of the time and continued to be pawns of powerful Roman families.
In a later post, I'll pick up the story in the eleventh century.
Click here for more on medieval Christianity.