Celtic Christianity was that form of the Christianity held by much of the population of the British Isles from about the end of the fourth century, until some time after the year 1171. Like any church it varied in form, from place to place, and time to time. However there is a constant stream that runs through that identified it as an unique entity. The classic period of Celtic Christianity ran from the fifth through the ninth centuries, in the “traditional” Celtic Lands (Ireland, Scotland, Wales, and Brittany) on the continent (France, Spain, Italy, Switzerland, and Germany) and beyond (Iceland, the Farroes, and other North Atlantic islands perhaps in Russia and North America).
Celtic Christianity was characterised by extreme holiness, a love of God and man, wanderlust and the need to bring to light of Christ to the world. Also, many of the issues that the Celtic Christians dealt with are amazingly contemporary, things like the position of women in the Church, nature and our environmental surroundings, and dealing with others of different customs and beliefs (both pagan and Christian). Much of its attraction comes from how it dealt with these problems, taking the best from older traditions while still standing firm in the truth.
Tradition holds that the faith was brought to the British Isles by Joseph of Arimathea and Aristobulus in A.D. 55 (some argue it was as early as A.D. 35) Modern scholarship rejects this, and places the introduction in the middle of the second century. Little is known of the first several centuries, however, Christianity was firmly established in Roman Britain by the time of the council of Arles (314) as two British bishops were in attendance. (There is also a possibility that British bishops were at Nicaea).
The true flowering of Celtic Christianity occurred after the Romans left Britain and they found themselves alone, surrounded by hostile barbarians. This is the time of the great celtic Saints: Patrick, David, Brigid, Columba, Brendan, Columbanus, and many, many others. This period was characterized by great holiness, love of learning and nature. It reached it’s peak in the seventh century in the Columban monastic federation of Iona. Its decline began soon after when, in 671, it lost Saxon Northumbria to the Roman observance.
This was by no means the end. Celtic Christianity survived the for the next five centuries. Due to many forces, demographic changes, Viking raids and settlement, and the expanding Roman rite; Celtic Christianity slowly retreated. Yet this was when the period that the Celts reached the pinnacle of their artistic genius; combining mediteranean plaitwork, barbarian zoomorphs, and their own native spiral and key patterns to create metalwork, illuminated manuscripts and stonecarving that amazes us even today. (Some examples include the Kells and Lindisfarne Gospels, the Ardagh Chalice, the Tara brooche and the Linsmore Croizer.)
Throughout the twelfth century, changes were pushed on the Irish system by eccesiastical reformers like Sts Malachi and Lawrence O’Toole. Finally in 1172, after the Anglo-Norman invasion, the Synod of Cashel ended the Celtic system. Even so, Celtic Christianity continued as an undercurrent in Celtic life. For the next four centuries the Culdees, monks of the Celtic tradition, retained some of the old ways as a minority rite. The last documented trace of them was in Armagh in 1628. Even into this century folk piety of the Scottish highlands and islands were strengthened by their Celtic Christian roots.
Source: The Celtic Christian List FAQ.