Main articles: Northumbria, Mercia, Offa of Mercia, Heptarchy, and Anglo-Saxon Christianity
Christianisation of Anglo-Saxon England began around 600 AD, influenced by Celtic Christianity from the northwest and by the Roman Catholic Church from the southeast. Augustine, the first Archbishop of Canterbury, took office in 597. In 601, he baptised the first Christian Anglo-Saxon king, Aethelbert of Kent. The last pagan Anglo-Saxon king, Penda of Mercia, died in 655. The last pagan Jutish king, Arwald of the Isle of Wight was killed in 686. The Anglo-Saxon mission on the continent took off in the 8th century, leading to the Christianisation of practically all of the Frankish Empire by 800.
Throughout the 7th and 8th century power fluctuated between the larger kingdoms. Bede records Aethelbert of Kent as being dominant at the close of the 6th century, but power seems to have shifted northwards to the kingdom of Northumbria, which was formed from the amalgamation of Bernicia and Deira. Edwin of Northumbria probably held dominance over much of Britain, though Bede's Northumbrian bias should be kept in mind. Succession crises meant Northumbrian hegemony was not constant, and Mercia remained a very powerful kingdom, especially under Penda. Two defeats essentially ended Northumbrian dominance: the Battle of the Trent in 679 against Mercia, and Nechtanesmere in 685 against the Picts.
The so-called "Mercian Supremacy" dominated the 8th century, though it was not constant. Aethelbald and Offa, the two most powerful kings, achieved high status; indeed, Offa was considered the overlord of south Britain by Charlemagne. That Offa could summon the resources to build Offa's Dyke is testament to his power. However, a rising Wessex, and challenges from smaller kingdoms, kept Mercian power in check, and by the early 9th century the "Mercian Supremacy" was over.
This period has been described as the Heptarchy, though this term has now fallen out of academic use. The word arose on the basis that the seven kingdoms of Northumbria, Mercia, Kent, East Anglia, Essex, Sussex and Wessex were the main polities of south Britain. More recent scholarship[by whom?] has shown that other kingdoms were also politically important across this period: Hwicce, Magonsaete, Lindsey and Middle Anglia.
source : wikipedia