The Ecclesiastical History of Sozomen

The church having been in this manner spread throughout the whole Roman world, religion was introduced even among the barbarians themselves. The tribes on both sides of the Rhine were Christianized, as likewise the Celts and the Gauls who dwelt upon the most distant shores of the ocean; the Goths, too, and such tribes as were contiguous to them, who formerly dwelt on either of the high shores of the Danube, had long shared in the Christian faith, and had changed into a gentler and more rational observance. Almost all the barbarians had professed to hold the Christian doctrine in honor, from the time of the wars between the Romans and foreign tribes, under the government of Gallienus and the emperors who succeeded him. For when an unspeakable multitude of mixed nations passed over from Thrace into Asia and overran it, and when other barbarians from the various regions did the same things to the adjacent Romans, many priests of Christ who had been taken captive, dwelt among these tribes; and during their residence among them, healed the sick, and cleansed those who were possessed of demons, by the name of Christ only, and by calling on the Son of God; moreover they led a blameless life, and excited envy by their virtues. The barbarians, amazed at the conduct and wonderful works of these men, thought that it would be prudent on their part, and pleasing to the Deity, if they should imitate those whom they saw were better; and, like them, would render homage to God. When teachers as to what should be done, had been proposed to them, the people were taught and baptized, and subsequently were gathered into churches.