From 'A History of the British Nation' by AD Innes, 1912
Gildas and Bede
Gildas, who wrote his book between 550 and 560, had very little knowledge of the English kingdoms, though he has much to say of the anarchy prevailing among the Britons. But from about this time Bede and the writers of the Anglo-Saxon Chronicle had more substantial records to deal with.
AEthelbert and AEthelric
The great King AEthelbert of Kent succeeded to the throne in 565, when Ceawlin, the first definitely historical figure in the Wessex records,was king of the West Saxons. Deira and Bernicia were still separate, but were to be united as Northumbria in 588 under AEthelric. The era of final conquest was now setting in Ceawlin at the moment was the most powerful of the southern kings; and after giving a check to AEthelbert of Kent and subjecting some of the Saxons on the north of the Thames to his sway, he turned his arms against the Britons, drove his way westward at the head of a force not so much of subjects as of confederates, and finally separated Damnonia from Wales by his great victory at Deorham, a few miles from Bath.
Thenceforth Saxons and Angles occupied the whole country as far west as the Severn valley, though the power was already departing from the crown of Wessex before Ceawlin died in 593. AEthelbert of Kent waxed great as Wessex weakened, and the eastern kingdoms acknowledged his supremacy as far north as the Humber.
AEthelfrith, King of United Northumbria after AEthelric, extended the Northumbrian dominion in the north, and in 613 shattered the allied forces of the Christian Celts at the battle of Chester, having ten years earlier utterly routed Aidan, the king of the Scots of Dalriada, who had gathered a large confederate army in the hope of crushing his rising power.
But Christianity had already obtained a footing among the southern English. The Britons never attempted missionary work among the conquerors. The Irish, Christianised in the fifth century, spread Christianity among the Celts of Scotland, and the contact with them first brought Christianity among the Angles of the north; but it was the mission of Augustine, organised by Gregory the Great himself, which introduced in the south the Latin Christianity which, in the course of the seventh century, dominated all England.
Augustine and his monks were well received by AEthelbert of Kent on their landing in 597; for AEthelbert's wife was already a Christian, being the daughter of one of the Merovingian kings of the Franks. The English seem nowhere to have had any very fervid attachment to their old paganism; there was never anything in the nature of a persecution of Christians. Christianity spread steadily and unglorified by martyrdoms. Unfortunately it did nothing towards reconciling the Britons and the English, because there were divergences on what seem to us extremely trivial points of practice between the Welsh and the Latin churches, and both sides obstinately refused to make any concessions.
Edwin of Northumbria
As supremacy passed from Wessex when Ceawlin grew old, so it passed from Kent when AEthelbert grew old. After his death in 616 Redwald of East Anglia enjoyed a temporary leadership, and even overthrew the Northumbrian conqueror, AEthelfrith, four years after the battle of Chester. He placed on the throne of Northumbria Edwin, the cousin of AEthelfrith, who had been ousted by AEthelric from the throne of Deira. Redwald died next year, and Edwin, now master of Northumbria, became the supreme king. Edwin was converted to Christianity, vanquished the kings who ventured to resist him, and appears to have enforced law and order to an unprecedented extent throughout the whole of his dominion, which extended north to Edinburgh or Edwin's borough. But there was one of the sub-kings in the midlands, Penda of Mercia, who was staunch to paganism, and was ready to defy the Northumbrian if opportunity offered. The Christian Welsh had no scruple in allying themselves with the old heathen, and Edwin was overthrown by Penda at the great battle of Heathfield. Penda of Mercia
Penda's Welsh allies ravaged Northumbria more mercilessly than Penda himself. The Northumbrians, however, rallied under Oswald, a son of AEthelfrith, and avenged Heathfield upon the Welsh at the battle of Hexham. Oswald partly recovered Edwin's supremacy over the island, but he never brought Penda to submission; and he, like his predecessor, was overthrown by the Mercian at Maserfeld in 642. After that the effective supremacy all over the island belonged to Penda until his death. It is a little confusing to find Oswald's brother Oswy ruling in Bernicia, while an Oswin of Edwin's line ruled in Deira, However, at last Oswy took heart of grace, defied Penda, and overthrew him at the battle of Winwaed, recovered the crown of Deira, and again established a general Northumbrian overlordship, though Penda had succeeded in consolidating the central kingdom of Mercia which remained in permanent rivalry with Northumbria. Penda himself was very nearly the last of the pagans, and his son Wulfhere was a Christian.
Oswy of Northumbria
Oswy's reign in Northumbria is especially notable on account of the synod held at Whitby in 664, nine years after the victory of Winwaed. Both Oswy and his predecessor Oswald had become Christians when they were dwelling among the Scots during the exile of their house. Hence Northumbrian Christianity was under the influence of the Celtic church. The outcome, however, of the open discussion held at the synod at Whitby was that Oswy resolved to conform to the Latin in preference to the Celtic practices; and this very much simplified the process, carried out under the Archbishop Theodore of Tarsus, of establishing the Latin ecclesiastical organisation under one primate all over England. The parish system
The six principal kings of Northumbria, Mercia, East Anglia, Essex, Wessex,and Kent had at first a bishop apiece; though Theodore divided each of the kingdoms into a larger number of dioceses, and Sussex, which had hitherto remained in pagan isolation, was brought into line with the rest. There is no sufficient ground for the tradition which attributes to Theodore the introduction of the ecclesiastical parish; but it is notable that the idea of English unity as one church preceded and helped to prepare the way for the idea of English political unity, which did not really take root until the days of Alfred. The Battle of Nectansmere
Oswy had extended some sort of ascendency over the Celtic dominion of Strathclyde, which marched with the western border of Northumbria from the Forth to the Mersey. But in 685, fourteen years after his death, Ecgfrith of Northumbria developed a too ambitious scheme of conquering the Pictish kingdom beyond the Forth. There he was enticed into the mountains, and his army was cut to pieces at the battle of Nectansmere, a blow from which Northumbria never recovered. By the opening of the eighth century the centre of greatest power was becoming established in Mercia.