Complex sentence: Celtic Christianity or Insular Christianity refers broadly to the Christianity of Britain and Ireland before and during the sub-Roman period of the Early Middle Ages, when the Roman withdrawal and the Anglo-Saxon invasion reduced contact with Continental Europe.
Split into simpler sentences: 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.