Sachsenkriege Karls des Großen
During the Saxon Wars in the 8th century the area round the Diemel was incorporated into the Frankish realm. Beside other places Warburg is presumed to be the location were the Irminsul, an old Saxon sacred pillar. The Austrian abbot Sturmius proselytized the area around the Diemel and Weser in 774. So the area around Warburg was Christianized from 774 on.