Little is known of the original form of Celtic spirituality (in Ireland, Scotland, Wales, England, and Brittany), which may have been influenced by druidic religion. It was dominated by a strict ascetic monasticism. Only an ordained monk in a monastery could become a bishop. An eclectic liturgy mixed Roman and Gallic rituals and kept the Jewish computation of Easter. Missionary zeal led to the founding of monasteries in continental Europe. Through the influence of Celtic spirituality, a monastic practice of individual confession and of satisfaction proportional to the offense became normative for the sacrament of penance in the whole western church. In 665 the Synod of Whitby abandoned Celtic customs for those of Rome. Yet great works of art (The Book of Kells, c. 775-800) testified to a persistent religious perception that was unique in Christendom. The monastic rule of St. Columba (c. 543-615) gave way gradually to the more moderate rule of St. Benedict.