This diocese came into existence in 635 when the great missionary Diocese of Dorchester, founded by St. Birinus in 634 for the Kingdom of Wessex,
was subdivided into the Sees of Sherborne and Winchester. The two dioceses were ruled by one bishop until 676, when a real separation was effected. The Diocese of Winchester then consisted of Hampshire, Surrey, and Sussex; but
Sussex was afterwards formed into the See of Chichester, and the Isle of Wight was added to Winchester. The church at Winchester, which became the cathedral of the new diocese, had been founded and endowed in 634 by King
Cynegils, whose son Coenwealh added more lands to its possessions. When Wessex gradually assumed the supremacy the importance of the see greatly increased. After the metropolitan Sees of Canterbury and York, it ranked first
among all English bishoprics till the reformation; this position the Anglican see still enjoys. It gained increased honour by the episcopate and subsequent canonisation of St. Swithin, its seventeenth bishop. When his relics
were enshrined there the cathedral, which had been under the patronage of St. Amphibalus, was dedicated to St. Swithin. It occupied the site of an earlier edifice dating from the Roman occupation, which had been converted into
a pagan temple by the Saxons.
A new cathedral was built by Cynegils, and three hundred years later was enlarged by Bishop Aethelwald, who replaced the secular canons by Benedictine monks and built a large monastery.
After the conquest the first Norman bishop, Walkelin, built a cathedral in the Norman style on a site near by; much of his work remains in the present edifice. To this new building (consecrated in 1093) the relics of St.
Swithin were solemnly transferred, 15 July. During the Middle Ages the building was gradually transformed from Norman to Gothic; the nave especially affords an interesting example of the way in which such changes were effected.
This work, began by Edington, was continued by the great bishop, William of Wykeham, and his successors.
Source: The Catholic Encyclopaedia, Volume XV