Between 1235 and 1802, the bishop of Hildesheim was also Prince of the Holy Roman Empire. His Hochstift (feudal princely territory) was the Prince-Bishopric of Hildesheim. In the 16th century, most of the diocese as well as most of the state of Hildesheim switched to Protestantism, but the bishopric managed to retain its independence from the surrounding Protestant states of Brunswick-Lüneburg, mostly because its bishops were members of the powerful House of Wittelsbach from 1573 until 1761.
Until 1824 the diocesan ambit remained unchanged, despite various changes of the political borders in history up to this date. After the Napoleonic wars the newly established Kingdom of Hanover stipulated with the Holy See to extend the Hildesheim diocesan ambit to all of the then Hanoverian territory east of the Weser river.[1] The newly included areas were Lutheran with a little Catholic diaspora and had formed part of the defunct dioceses of Bremen, of Mainz and of Verden before the Reformation.
Hannover's cession of land for Bremerhaven in 1827 to the prevailingly Reformed Bremen State did not alter the diocesan ambit. In 1834 the prevailingly Lutheran Duchy of Brunswick left the Apostolic Vicariate of Anhalt and agreed to extend Hildesheim's ambit to the ducal territory.[2] Thus the diocese covered areas in three sovereign states, with all of which and thus all the diocesan area becoming part of united Germany in 1871.
The incorporation of Hanoverian suburbs into Bremen city (Bremen North borough) in 1939 did not alter the ambit. In 1965 Hildesheim ceded that part of the then Hoya County District east of the Weser to the diocese of Osnabrück, whereas Osnabrück in return ceded Cuxhaven, Neuwerk, Scharhörn, Schaumburg-Lippe, as well as parts of the districts of Verden, Holzminden, Hameln-Pyrmont located west of the Weser, and the quarters of Nienburg upon Weser west of the river to Hildesheim.[3] In 1995 Hildesheim ceded its Harburg deanery in Hamburg south of the Elbe to the Archdiocese of Hamburg following the erection of this new see.[4]