Celtic Christianity or Insular Christianity is a term referring broadly to the Early Medieval Christian practice that developed in Britain and Ireland before and during the sub-Roman period. During this period, the Roman withdrawal and the Anglo-Saxon invasion sharply reduced contact between the broadly Celtic peoples of the islands and other Christians in Continental Europe. In this relatively isolated environment distinctive practices began to emerge, first gaining widespread use in Ireland in the 5th century (though elements of the Celtic Rite may have been introduced to Ireland by the British St. Patrick), and spreading to Britain with the Irish mission system established by Saint Columba. Celtic Christianity may be distinguished by its organisation around monasteries rather than dioceses, and certain traditions, especially in matters of liturgy and ritual, that were different from those of the greater sub-Roman world.