Liber Hymnorum

The term "Celtic Rite" is applied to the various liturgical rites used in Celtic Christianity in Britain, Ireland and Brittany and the monasteries founded by St. Columbanus and Saint Catald in France, Germany, Switzerland, and Italy during the early middle ages. The term does not imply homogeneity; the evidence, scanty and fragmentary as it is, is in favour of considerable diversity.

Before the 8th century AD there were several Christian rites in Western Europe. Such diversity of practice was often considered unimportant so long as Rome's primacy was accepted. Gradually the diversity tended to lessen so that by the time of the final fusion in the Carolingian period the Roman Rite, its Ambrosian variant, the Roman Rite and the Hispano-Gallican Mozarabic Rite were practically all that were left.

British bishops attended the Council of Arles in A.D. 314 and the Council of Rimini in 359. Communication with Gaul may be inferred from dedications to St. Martin at Withern and at Canterbury, from the mission of Victridius of Rouen in A.D. 396 and those of Germanus of Auxerre, with St. Lupus in 429 and with St. Severus in 447, directed against the Pelagianism of which the bishops of Britain stood accused.

However much of Britain derived their religion from Irish missionsaries. Aidan of Lindisfarne, Foillan, Diuma, Finan of Lindisfarne, Jaruman and others evangalised the Anglo-Saxons. Ia of Cornwall and her companions, Saint Piran, St. Sennen, Petroc came to Cornwall and probably brought with them whatever rites they were accustomed to. Cornwall had an ecclesiastical quarrel with Wessex in the days of St. Aldhelm, which appears in Leofric's Missal, though the details of it are not specified.

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Wikipedia