The modern Diocese of Hildesheim presently covers those parts of the state of Lower Saxony that are east of the River Weser, northern neighborhoods in Bremen, and the city of Bremerhaven.

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.