What role did the Roman Catholic Church play in European politics during the Middle Ages?

The separation of church and state is a recent concept, considering for centuries, religion and politics shared domains. In the Holy Roman Empire during the Middle Ages, the Roman Catholic Church was central to politics. The Pope and the church derived their power from God, and this established their absolute authority. Even the emperor was under the control of the Roman Catholic Church since the Pope had the power of excommunication. For example, in 1075, under the direction of Pope Gregory VII, the Dictatus Papae was instituted, establishing the supremacy of the Pope. This document granted the Papae the sole right to depose and reinstate bishops, pass judgment against clergy members, depose emperors, make new laws, and absolve subjects from their fealty (feudal allegiance). In 1076, control over politics resulted in conflict between King Henry IV and Pope Gregory VII. At this time, the practice of providing lay investiture (granting officials a church position in exchange for their fealty) connected politics and Christianity. However, this practice went against the stated rights of the Pope in the Dictatus Papae, and therefore, Pope Gregory VII declared it illegal. King Henry IV insisted on the right of lay investiture and wrote a demeaning, blasphemous letter to the Pope, who in turn excommunicated the king. Without backing from the Roman Catholic Church, no ruler could maintain power, so King Henry VII begged forgiveness, which was granted. Ultimately, though, the Roman Catholic Church asserted its power over the secular in politics.
Another example of the power of the Roman Catholic Church during the Middle Ages is the Crusades. The pope was responsible for encouraging Christians to take up the cross and fight against heretics in the Holy Land. The church had the power to convince everyday individuals to join the military pursuit of the empire in recapturing Jerusalem. By defining crusaders who gave their lives to the Christian cause as martyrs, the pope removed Christians sins for murder. Essentially, the Roman Catholic Church legalized murder and harm against anyone determined to be a heretic by the church. The Crusades demonstrated the political control of the Roman Catholic Church and its ability to manipulate subjects to defend and expand Christianity during the Middle Ages.

The Roman Catholic Church was very important during the Middle Ages. Kings and nobles had to listen to the Vatican and risked excommunication if they did not. This upset some princes especially in Germany, which is why the Protestant Reformation began in the German states. Kings had to give their armies and money to fight in various crusades. The pope was the only person in Europe who could make the decision to invade the Holy Land, though many kings enjoyed the fruits of their labors in attacking kingdoms in the Middle East.
The pope would issue edicts that Christians were not to kill one another so this led to jousting among knights instead of periods of constant warfare. This was important since in the early Middle Ages, the pope was the most powerful leader in Europe. While the Vatican sometimes found itself in conflict with various medieval leaders, the Roman Catholic Church was the most powerful European entity in the Middle Ages.