Saint Augustine of Canterbury Converted the Anglo-Saxons to Christianity
The Roman province of Britain had been largely Christian before it was overrun by the pagan Anglo-Saxons, whose conquered lands reverted to paganism. It was one of the few examples in history of a monotheistic faith getting rolled back from a territory in which it had gained a foothold, to be replaced by paganism. For more than a century after the Anglo-Saxon descent, the only predominately Christian areas in Britain were the lands still controlled by the indigenous Britons. Throughout the rest of the island, paganism was the dominant religious practice.
The re-Christianization of what had once been Roman Britain commenced in 565, when an Irish monk named Columba founded a monastery in the island of Iona, off the western coast of Scotland. That monastery began exerting a spiritual influence over the surrounding pagans, and Christianity gradually spread down the western coast of Scotland, and into northern Britain.
In 595, Pope Gregory the Great selected a Benedictine monk named Augustine, the prior of a monastery in Rome, to lead a mission of Christianization into the lands of the Anglo-Saxons. Augustine was sent to the kingdom of Kent, which dominated southwestern Britain and was ruled by a king Ethelbert, whose wife Bertha was a Christian. It was expected that she would aid the efforts to convert her husband and his people.
Bertha was the daughter of a Frankish king of Paris, and as one of the conditions of her marriage, had brought a bishop to Kent with her. Although a pagan himself, king Ethelbert allowed his wife freedom of worship, and the queen and her bishop restored a church in Canterbury that dated to Roman times. Thus, Christianity already had a toehold in Kent and the Kentish court when Augustine arrived in 597.
King Ethelbert allowed Augustine to preach in his capital of Canterbury, and within the year, Augustine had succeeded in converting the king. That led to the establishment of churches throughout Kent, and large scale conversions to Christianity. From Kent, Christianity spread to the neighboring Anglo-Saxon kingdoms in southern Britain. Augustine, considered an “Apostle to the English”, was later canonized as Saint Augustine of Canterbury, and is deemed to be the founder of the Catholic Church in England.
Farther to the north, a king Oswald of Northumbria asked the monastery of Iona in 635 to send a mission to Christianize his kingdom. Oswald had once been forced to flee Northumbria, and found refuge in the Christian enclaves of southwest Scotland. He converted, and determined to convert his kingdom upon regaining it. Iona sent him a monk named Aidan who could not speak English, but with the king acting as interpreter, the duo succeeded in converting the kingdom.
Oswald would eventually fall to king Penda of Mercia, when the latter rose to dominate the Anglo-Saxon kingdoms. After defeating and capturing the proselytizer king, Penda sacrificed Oswald to the pagan god, Woden. However, Christianity had already taken hold in Northumbria by then, thanks to Oswald, who ended up getting canonized as a saint.