Christianity
Christianity was introduced to Scotland in late Roman times, and traditions of the evangelizing of St. Ninian in the southwest have survived. He is a shadowy figure, however, and it is doubtful that his work extended very far north.
Firmly established throughout Scotland by the Celtic clergy, Christianity came with the Scots settlers from Ireland and possibly gave them a decisive cultural advantage in the early unification of kingdoms. The Celtic church lacked a territorial organization of parishes and dioceses and a division between secular and regular clergy; its communities of missionary monks were ideal agents of conversion. The best-known figure, possibly the greatest, is St. Columba, who founded his monastery at Iona, an island of the Inner Hebrides, in 565; a famous biography of his life was written by Adamnan, abbot of Iona, within a century of his death. Columba is believed to have been influential in converting the Picts, and he did much to support the Scots king Aidan politically.
St. Aidan brought the Celtic church to Northumbria in the 630s, establishing his monastery at Lindisfarne. At the Synod of Whitby in 664, the king of Northumbria, having to decide between the Celtic and the Roman styles of Christianity, chose the Roman version. There had been differences over such observances as the dating of Easter, but no one regarded the Celtic monks as schismatics. The Ecclesiastical History of the English People by Bede the Venerable (died 735), a monk of Jarrow in Northumbria, is a first-rate source for the early Anglo-Saxon history and shows remarkable sympathy with the Celtic clergy, though Bede was a Roman monk.
In the early 8th century the church among the Picts and Scots accepted Roman usages on such questions as Easter. Nevertheless, the church in Scotland remained Celtic in many ways until the 11th century. Still dominated by its communities of clergy (who were called Célidé or Culdees), it clearly corresponded well to the tribal nature of society.