Celtic Church, name given to the Christian Church of the British Isles before the mission (597) of St. Augustine of Canterbury from Rome. Founded in the 2d or 3d cent. by missionaries from Rome or Gaul, the church was well established by the 4th cent. when it sent representatives to the Synod of Arles (314) and to the Council of Rimini (359). It continued to spread in the 5th cent. due to the work of St. Ninian in Scotland, St. Dyfrig in Wales, and St. Patrick in Ireland. The heresies of the 4th cent. that played a significant role in church affairs on the Continent seem to have had little influence in Britain, and although it was the home of Pelagius (see Pelagianism), his teachings did not gain followers there until 421 with an influx of refugees from the Continent. The missions of St. Germanus of Auxerre (429 and 447) against the Pelagians in Britain and the spread of monasticism from Gaul attest to contacts with the church on the Continent. The Saxon invasions, beginning c.450, all but destroyed Celtic culture, dealing a deathblow to the Celtic Church in England through the destruction of the towns in which it had gained its greatest following. The few small Christian communities that survived were to be found in Wales and Ireland and in N and SW Britain. The period of peace that followed the British defeat of the Saxons at Mons Badonicus (c.500) once again allowed for growth of the Celtic Church (especially through the work of St. Columba), although isolation from the Continent continued until the mission of St. Augustine. Having converted King Æthelbert of Kent to Christianity, St. Augustine attempted to convince the leaders of the Celtic Church to change those practices (such as the dating of Easter and the forms of baptism and tonsure) that were at variance with the Roman Church and to accept the imposition of a diocesan organization on the essentially monastic structure of their church. He failed, and it was not until the Synod of Whitby (664, see Whitby, Synod of) that such agreement was largely reached, although independent Celtic churches continued on in Wales and Ireland.
See J. T. McNeil, The Celtic Churches (1974); F. E. Warren, The Liturgy and Ritual of the Celtic Church (1987).