Christianity came first to the Near East; its earliest adherents were in the Roman provinces of Palestine, Egypt and Ethiopia, Syria, Macedonia, and other places around the Mediterranean rim. In 301, Armenia formally adopted the Christian religion. During the next century, the Roman Empire also adopted it, and we can assume that the Gauls who lived in the Roman parts of modern France were presented with the new faith.
A surprising amount of the new faith’s transfer came through slavery. Wulfilas was born among the Goths, to Cappadocian Greek slaves in the Trans-Danube region around 310. His parents raised him in the Christian faith, but he was culturally Gothic otherwise and spoke at least three languages. He became a bishop among the Goths, translating parts of the Bible into Gothic (linguists are forever indebted to Wulfilas). Similarly, while Roman economic ties brought the new faith to the Celtic Gauls, we remember St. Patrick as the boy captured by the pagan Irish, enslaved so that he learned their language, and later returning as a free man to be a missionary and bishop. Other Roman-Britons had also brought the faith; we can’t put a date on its introduction to Ireland, but we know it was accomplished before about 400.
The other overlooked vehicle for transferring the new faith came from aristocratic intermarriages. In 490, one of the early Frankish kings, Clovis I, married a princess whose family had become Christian through contact with the Roman Empire. Clovis converted to her faith, with a large contingent of his war band. After the Angles, Saxons, and Jutes migrated/invaded into the island of England, King Aethelberht of Kent married a Frankish princess, Bertha, who brought a bishop to be her chaplain. There were probably many more such marriages.
The Visigoths and Lombards, who attacked different parts of Italy in the 500s, both were converted to a non-Roman form of Christianity, known now as Arianism. Perhaps to compete with the spreading Arianism, Pope Gregory the Great sent missionaries out to England in 596. This mission established a Roman Catholic church in Canterbury, in Aethelberht’s kingdom. Monasteries in Ireland also sent missionaries. From Christian England, missionaries crossed the Channel into the Low Countries and then to Saxony.
For the story of medicine, the important part is that in most European cases, conversion to Christianity meant the adoption of some portion of Roman culture. Ireland remained a non-Roman holdout for a long time, while the Lombards in Pavia were often in open war against Rome. But these were exceptions. During those same centuries, Rome had struggled to become dominant over Egypt, Syria, and Constantinople. While missionaries from Constantinople went into the eastern parts of Europe, Western Europe became a post-imperial colony of Christian Rome.
There were, then, (at least) three major belief systems present in early medieval Europe. First, the bedrock of their Celtic or Germanic pagan worldview. Second, the Roman worldview, which for a long time dominated science and medicine. Of course, Roman philosophy was largely based on the very advanced work of Greek culture a few centuries before. Third, the Christian Bible was Jewish in origin, and its worldview was different from either of these two.
So in tracing the story of medieval medicine, we need to look at all three ideas of how the world operates and who we are, as minds and bodies, within the world. The three systems competed and blended, finally creating the modern set of beliefs we inherited.