There were three major examples of the Medieval Christian Church controlling political and military behavior of Kings, and I think that, over time, the Church’s power grew. This is most exemplified by the Benedictine Monastery at Cluny and church reform, the First Crusade, and the Investiture Conflict of 1075.
First is the Benedictine Monastery at Cluny, founded in 910 by the Duke and Duchess of Aquitaine. The Monastery soon became rich after it received several gifts, pushing for the authority of the newly appointed Pope. The reason that so many people had such veneration and reverence for this specific Monastery is that it was given under the protection of Saint Peter and Saint Paul, should anybody ever choose to try to overrule it. The Benedictine Monastery at Cluny was just one of many examples of church reform, where people wished to strip the church of its ties to kings and nobles, so that they may be governed on their own. This was contrasted by the previous system, where churches were supported and protected by kings, in return for the ability to appoint bishops and have control over the Pope.
The First Crusade was possibly the most prominent example of church control. The church had been granting people “indulgences,” in return for those who chose to fight Muslims in Spain. Since the view of the public had shifted towards the importance of salvation, this was incredibly meaningful to the common man. Seljuk Turks (Sunni Muslims) began threatening the Byzantine territory, and the church was able to use that threat to mobilize people into fighting them, but at first, it didn’t really work. There was no real organization, but once they were able to coordinate large groups, they were able to defeat the Turks and capture Jerusalem.
This show of power helped prove that the church no longer needed military or political protection or support from kings and nobles. The last major example of the church’s control over kings is the Investiture Conflict of 1075. Essentially, Roman Emperor Henry IV wanted the ability to appoint bishops, but Pope Gregory VII kept insisting that, as the Pope, only he has that power, and it’s above Emperor Henry’s rule for him to do anything about it. Henry, on the other hand, argued that as King, he has power over the Pope, and should be able to do what he wants. Gregory saw himself as an incarnation of Saint Peter, stating that the King had no right to claim any authority over the church, Pope, or any of its holy associations.
Henry, on the other hand, saw himself as the authority over all of the land and subjects he ruled over, including the Pope. This was the Investiture Conflict, which ended in the excommunication of, and rebellion against, Emperor Henry IV.