Christianity probably came to Britain with the Roman legions, the spread of the faith being certainly helped by the infrastructure of the Roman Empire, resulting in the gradual conversion of the various Celtic peoples to the Christian faith. Thus a strong and lively Celtic church existed in Britain and Ireland before the Germanic invasions took place. We know that there were British bishops at church councils at Arles, 314 AD, and Rimini, 359 AD. There are records of the martyrdoms of Alban, Julius and Aaron. Such great numbers of Celts were converted that to be British and Celtic meant to be Christian. After the legions left there appear to have been some 150 years of warfare in Britain between the invading Anglo-Saxons and the original Celtic inhabitants. So when Augustine came from Rome in 596 he came into the conflict between the Anglo-Saxon conquerors and an indigenous church among a persecuted people.
A monk called Gildas writing c.550 describes the state of the British church and the Anglo-Saxon invasions. He describes the church as largely corrupt as a result of its contact with the world. Similar corruption took place in other parts of the Roman Empire and in Egypt some Christians sickened by the immorality of the cities and their impact on the church had fled into the desert to live as hermits. By c.320 groups of hermits were building simple homes for themselves in close proximity and so developed the early monasteries. The evidence would suggest that similar monastic communities came into Britain. The example seems to have come from Gaul but the inspiration from Egypt where many monks lived ascetic lives denying themselves marriage. These were not like the great monasteries of the Middle Ages, but probably collections of huts with a common meeting place or chapel. Thus Celtic Christianity took a form based on the monastery rather than the congregation and its leading figures were abbots rather than bishops. It maintained for a long time a spirit of bold independence in the face of the papacy’s claims to exercise authority over all Western churches as the one patriarchate of the West. It fostered a rare and intense love of the natural world of creation, beautifully expressed in Celtic prayers, hymns and poems. Some examples of early Celtic blessings:
May the road rise up before you,
may the sun be always on your face,
and the wind be to your back,
and until we meet again,
may the Lord keep you in the palm of his hand.
Deep peace of the running wave to you,
deep peace of the flowing air to you,
deep peace of the quiet earth to you,
deep peace of the shining stars to you,
deep peace of the Son of peace to you.
Much more at http://www.gracemagazine.org.uk/articles/historical/celtic.htm