In the days of Eadwine, Oswald, as the heir of the rival house of Bernicia, had passed his youth in exile, and had been converted to Christianity in the monastery of Hii, the island now known as Iona. The monastery had been founded by Columba, an Irish Scot. Christianity had been introduced into Ireland by Patrick early in the fifth century. Ireland was a land of constant and cruel war between its tribes, and all who wished to be Christians in more than name withdrew themselves into monasteries, where they lived an even stricter and more ascetic life than the monks did in other parts of Western Europe. Bishops were retained in the monasteries to ordain priests, but they were entirely powerless. Columba's monastery at Hii sent its missionaries abroad, and brought Picts as well as Scots under the influence of Christianity. Oswald now requested its abbot, the successor of Columba, to send a missionary to preach the faith to the men of North-humberland in the place of Paulinus, who had fled when Eadwine was slain. The first who was sent came back reporting that the people were too stubborn to be converted. "Was it their stubbornness or your harshness?" asked the monk Aidan. "Did you forget to give them the milk first and then the meat?" Aidan was chosen to take the place of the brother who had failed. He established himself, not in an inland town, but in Holy Island. His life was spent in wandering amongst the men of the valleys opposite, winning them over by his gentleness and his self-denying energy. Oswald, warrior as he was, had almost all the gentleness and piety of Aidan. 'By reason of his constant habit of praying or giving thanks to the Lord he was wont whenever he sat to hold his hands upturned on his knees.' On one occasion when he sat down to a feast with Aidan by his side, he sent both the dainties before him and the silver dish on which they had been served to be divided amongst the poor. "May this hand," exclaimed the delighted Aidan, "never grow old!"
In 627 Eadwine, moved by his wife's entreaties and the urgency of her chaplain, Paulinus, called upon his Witan to accept Christianity. Coifi, the priest, declared that he had long served his gods for naught, and would try a change of masters. 'The present life of man, O king,' said a thegn, 'seems to me in comparison of that time which is unknown to us like to the swift flight of a sparrow through the room wherein you sit at supper in winter, with your ealdormen and thegns, and a good fire in the midst, and storms of rain and snow without.... So this life of man appears for a short space, but of what went before or what is to follow we are utterly ignorant. If therefore this new doctrine contains something more certain, it seems justly to deserve to be followed.' On this recommendation Christianity was accepted. Paulinus was acknowledged as Bishop of York. The new See, which had been originally intended by Pope Gregory to be an archbishopric, was ultimately acknowledged as such, but as yet it was but a missionary station. Paulinus converted thousands in Deira, but the men of Bernicia were unaffected by his pleadings.
Christianity, like the extension of all better teaching, brought at first not peace but the sword. The new religion was contemptible in the eyes of warriors. The supremacy of Eadwine was shaken. The men of East Anglia slew their king, who had followed his over-lord's example by accepting Christianity. The worst blow came from Mercia. Hitherto it had been only a little state on the Welsh border. Its king, Penda, the stoutest warrior of his day, now gathered under him all the central states, and founded a new Mercia which stretched from the Severn to the Fens. He first turned on the West Saxons, defeated them at Cirencester, and in 628 brought the territory of the Hwiccas under Mercian sway. On the other hand, East Anglia accepted Eadwine's supremacy and Christianity. Penda called to his aid Cædwalla, the king of Gwynnedd, the Snowdonian region of Wales. That he should have done so shows how completely Æthelfrith's victory at Chester, by cutting the Kymric realm in two, had put an end to all fears that the Kymry could ever make head against England as a whole. The alliance was too strong for Eadwine, and in 633, at the battle of Heathfield—the modern Hatfield, in Yorkshire—the great king was slain and his army routed.