The explanation of that apparent contradiction is that this conversion which was brought about by St. Augustine was not lasting. The Mercians, who had been tributary, that is had paid tribute, to the Northumbrian king, allied themselves with the Britons of Wales and claimed independence. Their king Penda was the last of the great champions in England of the heathen gods, and his long reign was a continuous struggle against the new religion. By 650 he had defeated all his rivals except the {62} Northumbrians. Northumbria still held out against him, but St. Augustine's envoy, who had brought Christianity again to Northumbria, had departed after a victory gained by Penda over the Northumbrian king. Even in the south people relapsed in numbers into heathenism. The zeal for Christianity was kept alive in the north by influences that had come in through Ireland. From the Irish churches, untroubled by the incursion of barbarians, missionaries had come westward. A famous monastery had been established on the island of Iona, off the west coast of Scotland. Thence the missionary monks had passed on into Scotland, still, at that time, called Caledonia and inhabited by the people called Picts. They had passed, too, across the northern part of England and had settled on the island which even now is called Holy Island, off the east coast of Northumberland. That was the centre from which the new King of Northumbria and his people were inspired with a zeal for the Christian religion which made them continue the struggle against the Mercian king whose lordship was at this time acknowledged over most of the rest of Britain. Oswi, the Northumbrian king, had received some of his education at the monastery of Iona. In 655 he met and utterly defeated the Mercian forces, under the aged king Penda, near the modern town of Leeds. [Sidenote: Synod of Whitby]