[Illustration: ST. MARTIN'S CHURCH, CANTERBURY The present church, dating from the thirteenth century occupies the site of a chapel built before the arrival of Augustine, The walls still contain some of the Roman bricks used in the original structure. St Martin's Church was the scene of the earliest work of Augustine in Canterbury.] CELTIC CHRISTIANITY Augustine and his monks were not the first missionaries to Britain. Roman soldiers, merchants, and officials had introduced Christianity among the Britons as early as second century. During the fifth century the famous St. Patrick had carried Christianity to the heathen Irish. The Anglo-Saxon invasion of Britain drove many Christians to Ireland, and that island in the sixth and seventh centuries became a center from which devoted monks went forth to labor in western Scotland and northern Britain [27] Here they came in contact with the Roman missionaries. DIFFERENCES BETWEEN CELTIC AND ROMAN CHRISTIANITY The Celtic Christians followed some customs which differed from those observed by Roman Christians. They computed the date on which Easter fell according to a system unlike that of the Romans. They permitted their priests to marry; the Romans forbade the practice. Their monks shaved the front of the head from ear to ear as a tonsure, while Roman monks shaved the top of the head, leaving a "crown of thorns." These differences may not seem very important, but they were enough to prevent the cooperation of Celtic and Roman missionaries for the conversion of the heathen. [Illustration: CANTERBURY CATHEDRAL The choir dates from the twelfth century, the nave, transepts, and central tower, from the fifteenth century. One of the two towers at the west front was built in 1834-1840 A.D. The beautiful stained glass in the windows of the choir belongs to the thirteenth century.] SYNOD OF WHITBY, 664 A.D. The rivalry between Celtic and Roman Christians was finally settled at a church gathering, or synod, called by the king of Northumbria at Whitby. The main controversy at this synod concerned the proper date for Easter. In the course of the debate it was asserted that the Roman custom had the sanction of St. Peter, to whom Christ had intrusted the keys of heaven. This statement was enough for the Northumbrian king, who thereupon decided in favor of the Roman claim, declaring that he would not oppose St. Peter, "lest when I come before the gates of the kingdom of heaven, he who holds the keys should not open to me." [28] The representatives of the Celtic Church then withdrew from England, leaving the field clear for Roman missionaries. THE BRITISH ISLES BECAME ROMAN CATHOLIC