The Diocese of Winchester is one of the oldest and most influential in England. Originally it was the see of the kingdom of Wessex (as such it is sometimes called the "Diocese of Wessex"), with the first "cathedra" at lost Dorchester Cathedral which site is commemorated by later medieval Dorchester Abbey church in south-central Oxfordshire. The cathedral was founded and served successively by Saints Birinus and Agilbert, the first a missionary sent from Rome. This Wessex diocese not only covered most of Hampshire, Isle of Wight, Surrey, Berkshire, parts of Oxfordshire and Wiltshire but for the first few decades three more south-western counties mentioned below. The bishop's seat was swiftly transferred to Winchester in AD 660: the episcopal cathedral see was, at some point, at Old Minster, Winchester. Around 704–705, Aldhelm saw the four south-west peninsular counties of England, save for Cornwall, form the Diocese of Sherborne. To Devon, Somerset and Dorset, Cornwall was added at the end of the ninth century. These were well-settled and healthy counties in relative terms and in about 909 Sherborne was divided in three with the creation of the bishoprics of Wells, covering Somerset, and Crediton, covering Devon and Cornwall, leaving Sherborne comprising Dorset. Winchester shed north-western lands in AD 909 such that Wiltshire and Berkshire and the parts of Oxfordshire formed the See of Ramsbury, with its seat being Salisbury Cathedral.