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.