From Anglo Saxon England, by Sir Frank Stenton, ISBN 0-19-282237-3, page 9: The greatest of Anglo-Saxon historians was Bede. When tracing the genealogy of Æthelbert, the first English king to become a Christian, he observes that the kings of Kent in his own time were known as the 'Oiscingas' because Eric, Hengest's son, from whom they were descended, had borne the surname Oisc.

From "Debrett's Kings and Queens of Britain" by Williamson, ISBN 0-86350-101-X, p 15:
The first Anglo-Saxon kingdom to be founded was also the first to receive Christianity. Ethelbert's wife was a Christian and was allowed to bring her chaplain to Kent with her and practice her religion. Pope Gregory the Great despatched the monk Augustine to England. King Ethelbert already knew something of Christianity from his wife and agreed to receive Augustine and his party and hear what they had to say. He was an easy convert, being half-way there already, and granted Augustine a residence in the Royal City of Canterbury, of which Augustine became the first Archbishop after visiting France for consecration by the Archbishop of Arles. King Ethelbert died on 24 Feb 616. Bertha had predeceased him and he had married again.
p 14: Later in the 5th century, other settlers established kingdoms throughout England, The kingdoms so established are known as the heptarchy (i.e. the 7 kingdoms, although the number varied). Although the kingdoms were quite independent of each other, they formed a loosely knit confederation under the leadership of one king, usually the most powerful among them, who was elected as a sort of head king with the title of Bretwalda. The seven Bretwaldas listed by Bede were:
Aella, King of the South Saxons (Sussex)
Caewlin, King of the West Saxons (Wessex)
Æthelbert I, King of Kent
Redwald, King of the East Angles
Edwin, King of Northumbria
Oswald, King of Northumbria
Oswy, King of Northumbria.

His wife, Bertha, daughter of the Frankish king Charibert was a Christian which may explain why Pope Gregory's missionaries were sent initially to Kent to convert the Anglo-Saxons to Christianity. After his baptism by St Augustine, Aethelbert permitted the erection of a cathedral and a monastery at Canterbury. The cathedral is still the site of the head of the Church of England. Bede suggests that there were seven great kings who ruled outside their own kingdoms during Anglo-Saxon times: Aethelberht is listed third. Ethelbert (552?-616),
Anglo-Saxon king of Kent (560-616). In 590, after many battles, he was recognized as Bretwalda, or ruler of the Britons. He married Bertha, a Frankish princess of the Christian faith, and was himself converted to Christianity and baptized by St. Augustine of Canterbury in 597. Shortly afterward he had episcopal sees created at Canterbury (his capital), Rochester, and London. Æthelbert promulgated a Saxon code of laws, the first of its kind, based on Roman law. Source: "Ethelbert," Microsoft(R) Encarta(R) 98 Encyclopedia. (c) 1993-1997 Microsoft Corporation.