Bede's Ecclesiastical History of England, ed. by A.M. Sellar, , at sacred-texts.com
IN the year of our Lord 565, when Justin, the younger, the successor of Justinian, obtained the government of the Roman empire, there came into Britain from Ireland a famous priest and abbot, marked as a monk by habit and manner of life, whose name was Columba,to preach the word of God to the provinces of the northern Picts, who are separated from the southern parts belonging to that nation by steep and rugged mountains. For the southern Picts, who dwell on this side of those mountains, had, it is said, long before forsaken the errors of idolatry, and received the true faith by the preaching of Bishop Ninias, a most reverend and holy man of the British nation, who had been regularly instructed at Rome in the faith and mysteries of the truth; whose episcopal see, named after St. Martin the bishop, and famous for a church dedicated to him (wherein Ninias himself and many other saints rest in the body), is now in the possession of the English nation. The place belongs to the province of the Bernicians, and is commonly called the White House, because he there built a church of stone, which was not usual among the Britons.
Columba came into Britain in the ninth year of the reign of Bridius, who was the son of Meilochon, and the powerful king of the Pictish nation, and he converted that nation to the faith of Christ, by his preaching and example. Wherefore he also received of them the gift of the aforesaid island whereon to found a monastery. It is not a large island, but contains about five families, according to the English computation; his successors hold it to this day; he was also buried therein, having died at the age of seventy-seven, about thirty-two years after he came into Britain to preach.Before he crossed over into Britain, he had built a famous monastery in Ireland, which, from the great number of oaks, is in the Scottish tongue called Dearmach—The Field of Oaks. From both these monasteries, many others had their beginning through his disciples, both in Britain and Ireland; but the island monastery where his body lies, has the pre-eminence among them all.
That island has for its ruler an abbot, who is a priest, to whose jurisdiction all the province, and even the bishops, contrary to the usual method, are bound to be subject, according to the example of their first teacher, who was not a bishop, but a priest and monk;of whose life and discourses some records are said to be preserved by his disciples. But whatsoever he was himself, this we know for certain concerning him, that he left successors renowned for their continence, their love of God, and observance of monastic rules. It is true they employed doubtful cycles in fixing the time of the great festival, as having none to bring them the synodal decrees for the observance of Easter, by reason of their being so far away from the rest of the world; but they earnestly practiced such works of piety and chastity as they could learn from the Prophets, the Gospels and the Apostolic writings. This manner of keeping Easter continued among them no little time, to wit, for the space of 150 years, till the year of our Lord 715.
But then the most reverend and holy father and priest, Egbert, of the English nation, who had long lived in banishment in Ireland for the sake of Christ, and was most learned in the Scriptures, and renowned for long perfection of life, came among them, corrected their error, and led them to observe the true and canonical day of Easter; which, nevertheless, they did not always keep on the fourteenth of the moon with the Jews, as some imagined, but on Sunday, although not in the proper week.For, as Christians, they knew that the Resurrection of our Lord, which happened on the first day of the week, was always to be celebrated on the first day of the week; but being rude and barbarous, they had not learned when that same first day after the Sabbath, which is now called the Lord’s day, should come. But because they had not failed in the grace of fervent charity, they were accounted worthy to receive the full knowledge of this matter also, according to the promise of the Apostle, "And if in any thing ye be otherwise minded, God shall reveal even this unto you." Of which we shall speak more fully hereafter in its proper place.