This is a term of convenience generally used to describe the ecclesiastical practices and structures which were to be found in the Celtic areas of the British Isles and Brittany (those areas in which P- and Q-Celtic languages were spoken in the period c. 400 to 1100 AD). It is misleading in a number of respects. The main problem is that it implies a single church, when in fact there was no such uniformity. The concept of a Celtic unity, deriving from a shared language or culture or faith, would have been unacknowledged, and indeed unknown, in this period, and there was no single metropolitan with power over the churches in all the Celtic areas. The term is also often taken to imply that the ‘Celtic Church’ was a quite different entity from the ‘Roman Church’, not only in structure but also in belief. This too is incorrect. Although the churches in the Celtic areas did differ from Rome on matters such as the dating of Easter, the form of tonsure, and baptism, there were no major differences in matters of doctrine, and the churches of the Celtic areas were in touch with Rome.
Much of the popular understanding of the ‘Celtic Church’ is based on what is known about the Gaelic church, which was to be found in Ireland and Scotland, but it is best to distinguish this church from that in Wales and Brittany by using the appropriate labels for each body -Gaelic, Welsh or Breton. There were nevertheless common factors. All the churches in these lands gave a significant place to monasticism; they were closely associated with secular society, and derived much of their resources from donations made by sympathetic landowners; and they used a common language, namely Latin, in addition to the vernacular Celtic languages. These points are all evident in the contents of the Book of Deer. In addition to fulfilling their religious duties, monks in these early churches were devoted to learning, and contributed greatly to the cultural development of Ireland, Scotland and Wales. They set great store by literacy, and recorded in writing the earliest literary texts to be found in the Celtic countries. The arts and crafts were developed within these churches to enhance Christian devotion by means of finely carved crosses, furnishings and ornaments, and illuminated manuscripts. Deer also reflects this tradition.
The Christian faith itself was probably introduced to the British Isles in the Roman period, although no precise date or place of entry can be given. Patrick, commonly regarded as ‘The Apostle of Ireland’, who probably flourished in the fifth century, may have come from a romanised Christian community in Britain. He was captured by pirates and taken to Ireland. He later returned to Ireland to proclaim the faith there, but it is very unlikely that he was the first Christian ‘missionary’ active in Ireland. It is likely that Christianity was introduced to Ireland through various channels, including links with Britain, Gaul and the Mediterranean. The expression of the faith in Ireland reflected eastern practices, particularly in its emphasis on the ascetic life. Gaelic monks, including some of the best known Saints, were often severely ascetic. They travelled widely and established monasteries, like that of Colum Cille (Columba) in í (Iona).
The churches in the Celtic areas were reformed periodically. The reform movement of the céili Dé (clients of God) arose in Ireland in the eighth century as a reaction against the secularisation of the church. It influenced Scotland, where the term was scotticised as ‘Culdee’. From the twelfth century the existing Gaelic religious bodies, including the Culdees, were gradually absorbed into, or replaced by, the medieval monastic orders.