Dictionary of National Biography, 1885-1900/Sigebert (fl.653)
SIGEBERT or SEBERT, called the Good (fl. 653), king of the East-Saxons, was son of Sigebald, who was descended from Seaxa, the brother of Sebert or Saberet (d. 616?) [q. v.], king of the East-Saxons. He succeeded his kinsman Sigebert or Sebert, called the Little (fl. 626) [q. v.], probably through the support of Oswy [q. v.], king of the Northumbrians. At Oswy's persuasion he became a Christian, and was baptised, together with his followers, by Bishop Finan [q. v.] at a place called At-Wall, near the Roman wall, in or about 653. In accordance with his request that teachers might be sent to his kingdom to convert his people, who had remained heathen since their apostasy after the death of Sebert, in or about 616, Oswy summoned Cedd or Cedda [q. v.] from the land of the Middle-Angles, and sent him with another priest to preach to the East-Saxons. They were successful in their mission, and Cedd, having been consecrated bishop of the East-Saxons, made Ythanceaster—said to have been near Malden and Tilaburg (Tilbury), both in Essex—centres for his work, which, Bishop Stubbs remarks, makes it probable that London lay outside Sigebert's kingdom, and was under Mercian rule, as it certainly was a little later (Bede, Hist. Eccl. iii. c. 7). The story of the rebuke that Sigebert received from Cedd, and the bishop's prophecy, has been told elsewhere [see under Cedd or Cedda, Saint]; but it may be added that the two brothers, his kinsmen, who slew Sigebert, declared that they had done so because they were indignant with him for sparing his enemies and bearing injuries placidly—probably referring to the king's humiliation before Cedd. Bede dwells on the fact that his death was caused by his obedience to Christian principles, and highly commends his piety. The date of his death, which is not known, has been assumed to be 660 (Hist. Eccl. ed. Stevenson, p. 209; Monumenta Hist. Brit. pp. 195–6). But Bishop Stubbs thinks that it probably occurred before the battle of Winwæd in 655; and if Sigebert's successor, Swithelm, was baptised on his accession, which from Bede's account seems likely, he is undoubtedly right. On the other hand, Bede says that Sigebert's death took place a short time after his baptism.
[Bede's Hist. Eccles. iii. c. 22; Mon. Hist. Brit. pp. 629, 637; Will. Malm. Gesta Regum, i. c. 98; Dict. Chr. Biogr. art. ‘Sigebert’ (6), by Bishop Stubbs.]