The Christian Church in the British Isles from its foundation in the 2nd or 3rd century until its assimilation into the Roman Catholic Church (664 in England; 12th century in Wales, Scotland, and Ireland).
Definition of Celtic Church in US English:
Celtic Church
proper noun
The Christian Church in the British Isles from its foundation in the 2nd or 3rd century until its assimilation into the Roman Catholic Church (664 in England; 12th century in Wales, Scotland, and Ireland).