In less than a hundred years after the arrival of St. Augustine, England became Christian, and the conversion had been in many cases accomplished by missionaries from Rome. But many of the kingdoms also had been brought to the new faith by bishops from Scotland; Mercia, for instance, and Northumbria finally. These bishops came in many cases from the island of Iona, and they did not acknowledge or follow the customs of the Church of Rome. The great question as to which of the two rituals should prevail was settled at a synod held at Whitby, when Northumbria adopted the Roman use, and from that time ecclesiastical unity prevailed. The organisation of the Church of England was effected by Theodore of Tarsus, who was sent over to England as Archbishop of Canterbury in 668. He proceeded to organise the various sees, usually following the limits of the old English kingdoms; and though changes were occasionally made, much of his work was permanent, and exists at the present day. So England was one kingdom as far as its religious constitution was concerned, and this unity led in turn, as we shall see, to a civil unity under the kings of Wessex. By the beginning of the eighth century it had become evident that the struggle for supremacy would eventually be between Wessex and Mercia, for Northumbria, a turbulent state, harassed by succession questions, had already ceased to hold the pride of place. At first Mercia appeared to have the advantage of the struggle. It soon recovered from the overthrow of Penda, and from the years 716 to 819, with one or two intervals of temporary prostration, it was extremely powerful. Ethelbald, the nephew of Penda, reigned from 716 to 755, and built up a great power. Taking advantage of the anarchy in Northumbria, and of the abdication of Ina of Wessex, he subdued his neighbours in a series of successful wars, and claimed to be king "not only of the Mercians, but of all the people who are called by the common name of South-Angles." He was, however, in 754, confronted by a general rebellion, and utterly defeated in a battle at Burford.