Exile, Conversion, and Return
While in Gaul during his long exile, Sigeberht underwent Christian conversion and baptism and became devoted to learning, being strongly impressed by the religious institutions and the schools for the study of reading and writing that he encountered there.
Following the assassination of Eorpwald around 627, Sigeberht returned to East Anglia and became king. He appears to have gained the kingdom by military means, as his prowess as a commander was later remembered. He ruled jointly with Ecgric, described as his kinsman (Latin: cognatus), who may have been either a son or a nephew of Rædwald.
His Christian faith proved strategically advantageous, aligning him with Edwin of Northumbria and Eadbald of Kent, both Christian rulers of significant power.
Establishing the Church and Schools
Sigeberht established Christianity throughout East Anglia with the assistance of Saint Felix of Burgundy, who had been sent by Archbishop Honorius of Canterbury. Felix became the region's first bishop, with his see at Dommoc (variously identified as Dunwich, Walton, or Felixstowe).
On the model he had witnessed in Gaul, Sigeberht established a school in his kingdom so that boys could be taught reading and writing in Latin; Felix obtained teachers from Kent to staff it.
Around 633 Sigeberht granted the Irish monk Saint Fursey a monastery site in an old Roman fort called Cnobheresburg, typically identified as Burgh Castle near Great Yarmouth. Fursey and Felix together effected a large number of conversions and established many churches in the kingdom. The church Sigeberht helped to found survived roughly two centuries under successive bishops until the Danish Great Heathen Army invaded East Anglia in the 860s.
Abdication and Martyrdom
At some point during his reign Sigeberht abdicated his secular powers to Ecgric in order to pursue the religious life, retiring to a monastery later identified as Beodricesworth (modern Bury St Edmunds) in the Lark valley.
At an unknown date, possibly the early 640s, a Mercian army attacked East Anglia. Despite his monastic vows, Sigeberht was dragged from the monastery to the battlefield. Bede records that he went into battle carrying only a staff, refusing to bear arms.
Sigeberht, Ecgric, and many of the East Angles were slain and their army routed, and he came to be honoured as a Christian martyr. According to tradition he was among the kings later avenged by the slaying of Penda in 654.
