The Mission of the Church can best be understood from Paul’s address to the Athenians in Greece in Acts 17:22-23 when he pointed to a statue labeled "the unknown God" and informed the Greeks that they were already worshipping the God he now named as Jesus. As Christianity was spreading throughout the Roman Empire, it often used the Greek ideas and language to describe Jesus. But over time with the Roman leadership that approach was forced to change.

Islam in the 7th and 8th centuries began to take hold after Mohammad’s death in 632 AD. Islam was sweeping rapidly across the Roman Empire’s former eastern, southern and western provinces. Islam religion was in control of the Middle East, north Africa and the Iberian peninsula. Rome was forced to look north for a protector and for evangelization opportunities. They turned to Europe. Monks from Ireland, Britain and Germany played a key role in the implementation of a strategy discovering where people's faith already was operating, even if it was still a mix of Roman paganism and Arianism, which was often the case in central and northern Europe. One example is St. Patrick, a 5th-century Briton who was successful in converting the Irish precisely because he had lived among them as a captive for six years.

Missionaries were used to further the spread of Christianity in pagan sites rather than destroy them. Those who were already inclined to believe in the healing power of water at a particular site would now find that a baptistery at a well or bend in the river as providing this healing process. Pagan temples were sprinkled with holy water converting them to altars.

These and other ways used by missionaries quickly spread Catholicism successfully across Visigothic Spain, Merovingian and Carolingian Gaul, and Anglo-Saxon England before turning east to Poland, Hungary, and Russia in the closing decades of the first millennium. Moreover, there was a measure of liturgical variation in the Middle Ages that was eventually regulated in 1570, with the Roman Missal issued shortly after the Council of Trent. A precise order for Mass was laid down, but variations were permitted as long as they were at least 200 years old, which was the case with the Ambrosian rite in Milan and Spain's Mozarabic rite, both dating to the middle of the first millennium.

Much of the Church's evangelization and enculturation efforts in the first 1500 years of Catholicism were increasingly driven by an emerging, then strengthening papacy that was concentrating power and authority in Rome, culminating in the creation of a papal monarchy in the Middle Ages. In the Church's first few centuries, the bishop of Rome enjoyed no special jurisdictional status among his many brother bishops across the Mediterranean world, although there was a certain recognition of his unique place as Peter's successor.

When the Roman Empire fell we find that instead of the church turning to God for their government they turned inward to themselves and sought what they felt they needed to do for protection and security. In doing so, laid the foundation to what changed the church forever. Well not changed but solidified its foundational core belief. When the Roman Empire fell the bishop of Rome was forced by circumstance to take on a civil or secular role to maintain safety and the orderly running of the city of Rome and the nearby central portion of the Italian peninsula. So it was Pope Leo I who in 452 and 455 rode out to bribe the Huns and Vandals into skirting Rome. About 150 years later, Pope Gregory I complained that he spent too much time as a paymaster administering property and overseeing the local army, illustrating the extent to which the papacy administered in a secular sense.

Because of Islam's spread, popes seeking protection turned north, most notably to Charlemagne, who was only too happy to take on Constantine's mantle as defender of the faith and Church. Pope Leo III crowned him the Holy Roman Emperor in 800, thus beginning centuries of an uneasy alliance between royal powers who saw themselves in a theocratic role as sacred kings, which was the model in the Byzantine Empire, and popes who had sometimes to be civil rulers but who always claimed that spiritual power trumped temporal power.

In order to compete and as part of the effort to evangelize and centralize Catholicism, popes starting in the middle of the 11th century broke away from the cozy relationship with royal power and set up a rival monarchy complete with a law system (canon law) and cou