Christianity in the Middle Ages covers the history of Christianity from the fall of the Western Roman Empire (c. 476). The end of the period is variously defined - depending on the context, events such as the conquest of Constantinople by the Ottoman Empire in 1453, Christopher Columbus's first voyage to the Americas in 1492, or the Protestant Reformation in 1517 are sometimes used.

In Christianity's ancient Pentarchy, five patriarchies held special eminence: the sees of Rome, Constantinople, Jerusalem, Antioch, and Alexandria. The prestige of most of these sees depended in part on their apostolic founders, or in the case of Byzantium/Constantinople, that it was the new seat of the continuing Eastern Roman, or Byzantine Empire. These bishops considered themselves the successors of those apostles. In addition, all five cities were early centres of Christianity, losing their importance after the Levant was conquered by the Sunni Caliphate.

The Early Middle Ages commenced when the last western Roman emperor was deposed in 476, to be followed by the barbarian king, Odoacer, to the coronation of Charlemagne as "Emperor of the Romans" by Pope Leo III in Rome on Christmas Day, 800. The year 476, however, is a rather artificial division. In the East, Roman imperial rule continued through the period historians now call the Byzantine Empire. Even in the West, where imperial political control gradually declined, distinctly Roman culture continued long afterwards; thus historians today prefer to speak of a "transformation of the Roman world" rather than a "fall of the Roman Empire."

The advent of the Early Middle Ages was a gradual and often localised process whereby, in the West, rural areas became power centres whilst urban areas declined. With the Muslim invasions of the seventh century, the Western (Latin) and Eastern (Greek) areas of Christianity began to take on distinctive shapes. Whereas in the East the Church maintained its strength, in the West the bishops of Rome (i.e., the Popes) were forced to adapt more quickly and flexibly to drastically changing circumstances. In particular whereas the bishops of the East maintained clear allegiance to the Eastern Roman Emperor, the bishop of Rome, while maintaining nominal allegiance to the Eastern Emperor, was forced to negotiate delicate balances with the "barbarian rulers" of the former Western provinces. Although the greater number of Christians remained in the East, the developments in the West would set the stage for major developments in the Christian world during the later centuries.

After the Italian peninsula fell into warfare and turmoil due to the barbarian tribes, the Emperor Justinian I attempted to reassert imperial dominion in Italy from the East, against the Gothic aristocracy. The subsequent campaigns were more or less successful, and an Imperial Exarchate was established for Italy, but imperial influence was limited. The Lombards then invaded the weakened peninsula, and Rome was essentially left to fend for itself. The failure of the East to send aid resulted in the popes themselves feeding the city with grain from papal estates, negotiating treaties, paying protection money to Lombard warlords, and, failing that, hiring soldiers to defend the city. Eventually the popes turned to others for support, especially the Franks.

As the political boundaries of the Roman Empire diminished and then collapsed in the West, Christianity spread beyond the old borders of the Empire and into lands that had never been under Rome.

Beginning in the fifth century, a unique culture developed around the Irish Sea consisting of what today would be called Wales and Ireland. In this environment, Christianity spread from Roman Britain to Ireland, especially aided by the missionary activity of St. Patrick with his first-order of 'patrician clergy', active missionary priests accompanying or following him, typically Britons or Irish ordained by him and his successors. Patrick had been captured into slavery in Ireland and, following his escape and later consecration as bishop, he returned to the isle that had enslaved him so that he could bring them the Gospel. Soon, Irish missionaries such as Columba and Columbanus spread this Christianity, with its distinctively Irish features, to Scotland and the Continent. One such feature was the system of private penitence, which replaced the former practice of penance as a public rite.

Although southern Britain had been a Roman province, in 407 the imperial legions left the isle, and the Roman elite followed. Some time later that century, various barbarian tribes went from raiding and pillaging the island to settling and invading. These tribes are referred to as the "Anglo-Saxons", predecessors of the English. They were entirely pagan, having never been part of the Empire, and although they experienced Christian influence from the surrounding peoples, they were converted by the mission of St. Augustine sent by Pope Gregory the Great. The majority of the remaining British population converted from Christianity back to their Pagan roots. Contrary to popular belief, the conversion of Anglo-Saxons to Christianity was incredibly slow. The Anglo-Saxons had little interest in changing their religion and even initially looked down upon Christianity due to conquering the Christian British people decades earlier.