Ancient Celtic religion
Christianisation

Celtic societies under Roman rule presumably underwent a gradual Christianisation in similar ways to the rest of the Empire; there is next to nothing in Christian sources about specific issues relating to Celtic people in the Empire, or their religion. Saint Paul 's Epistle to the Galatians was addressed to a congregation that might have included people from a Celtic background.

In Ireland, the main Celtic country unconquered by the Romans, the conversion to Christianity (Christianisation) inevitably had a profound effect on the socio-religious system from the 5th century onward, though its character can only be extrapolated from documents of considerably later date. By the early 7th century the church had succeeded in relegating Irish druids to ignominious irrelevancy, while the filidh, masters of traditional learning, operated in easy harmony with their clerical counterparts, contriving at the same time to retain a considerable part of their pre-Christian tradition, social status, and privilege. But virtually all the vast corpus of early vernacular literature that has survived was written down in monastic scriptoria, and it is part of the task of modern scholarship to identify the relative roles of traditional continuity and ecclesiastical innovation as reflected in the written texts.

Cormac's Glossary (c. 900 AD) recounts that St. Patrick banished those mantic rites of the filidh that involved offerings to "demons", and that the church took particular pains to stamp out animal sacrifice and other rituals repugnant to Christian teaching. What survived of ancient ritual practice tended to be related to filidhecht, the traditional repertoire of the filidh, or to the central institution of sacral kingship. A good example is the pervasive and persistent concept of the hierogamy (sacred marriage) of the king with the goddess of sovereignty: the sexual union, or banais ríghi ("wedding of kingship"), which constituted the core of the royal inauguration, seems to have been purged from the ritual at an early date through ecclesiastical influence, but it remains at least implicit, and often quite explicit, for many centuries in the literary tradition.
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