Thor's Oak was an ancient tree sacred to the Germanic tribe of the Catti, ancestors of today's Hessians, and one of the most important sacred sites of the Germans. The tree stood at a location near the village of Geismar, today part of the town of Fritzlar, and was the main point of veneration of the Germanic deity Thor by the Catti and most other German tribes. Its felling in 723 marked the beginning of the Christianization of the non-Frankish Germans.

In 723, the Anglo-Saxon missionary Winfrid -- later called St. Boniface, Apostle of the Germans -- arrived in the area in his quest to convert the northern German tribes, using the nearby Frankish fortified settlement of Büraburg as his base. He had the oak felled to prove the superiority of the Christian god over Thor and the Germanic deities, and when Thor did not respond by hurling a lightning bolt at him, the assembled local people agreed to be baptized. Boniface used the wood of the oak to build a chapel in Fritzlar, founded a Benedictine monastery, and established the first bishopric in Germany at Büraburg, located on a prominent hill opposite Fritzlar across the Eder river. The first abbot of Fritzlar, St. Wigbert, built a stone basilica at the site of the wooden chapel which was, after its destruction by Saxon rebels in 1079, replaced in 1180-1200 by the large Romanesque-Gothic cathedral of St. Peter that today dominates the town.

The bishopric of Büraburg was not renewed after the death of Witta in 747, the first and only bishop of Büraburg, but was incorporated instead into the bishopric (later archbishopric) of Mainz by Lullus, Boniface's successor as archbishop of Mainz.