John is the medieval and modern English form of the biblical name rendered Johannes or Iohannes in Latin texts, including Domesday Book. It first arrived in England with the Gregorian mission (PASE: John 5) and was used in clerical circles to some degree both in the pre-Viking period (e.g. PASE: John 18, 19, and 22) and in the tenth and eleventh centuries (Searle 1897: 319), probably as a name taken in religion by men who had been christened with Old English names from the usual repertoire. Most of the persons called John who figure in Anglo-Saxon history were popes and other clerics and monks from the Continent. As with other biblical names, John was not used among the English laity, unless it was the name behind Johannan, a slave at Faccombe (Hants) freed by his mistress in the late tenth or early eleventh century (PASE: Johanna 1; S 1539).
Other than the abbot of Fécamp, all the pre-Conquest instances of John in DB seem to be one person, John the Dane. It is almost as surprising to find it as the name of a Danish layman as it would be of an Englishman.
Searle 1897: William George Searle, Onomasticon Anglo-Saxonicum: A List of Anglo-Saxon Proper Names from the Time of Beda to that of King John (Cambridge: Cambridge University Press, 1897)
Spellings in Domesday Book: Johannes
Spellings in Exon: Iohannes
Forms in modern scholarship:
von Feilitzen head forms: Johannes
Phillimore edition: John
Alecto edition: John