The Prince-Bishopric must not be confused with the Diocese of Hildesheim, which was larger and over which the prince-bishop exercised only the spiritual authority of an ordinary bishop.

After the Duchy of Saxony had been conquered by the Frankish Kingdom, Emperor Charlemagne in 800 founded a missionary diocese at his eastphalian court in Elze (Aula Caesaris), about 19 km (12 mi) west of Hildesheim.

According to legend delivered by the Brothers Grimm, the king was hunting in the wintery woods of Elze, when he realized that he had lost his pendant with the relic of Blessed Virgin Mary.

[3] During the reign of the Saxon Ottonian dynasty Hildesheim, together with the neighbouring bishoprics of Halberstadt and Magdeburg, became the central ecclesiastical territory of the Holy Roman Empire.

But the Bishopric managed not only to retain its independence from the surrounding Protestant states of Brunswick-Lüneburg, but also to retrieve large parts of the lost estates, mostly because its bishops were members of the powerful House of Wittelsbach from 1573 until 1761, the last Clemens August of Bavaria from 1723, who also was archbishop and prince-elector of Cologne, prince-bishop of Münster, Osnabrück and Paderborn as well as Grand Master of the Teutonic Order.