Question: "What was the Investiture Controversy, and how did it impact Christianity?"
Answer: The Investiture Controversy, also called the Investiture Contest or the Investiture Dispute, lasted from AD 1076 to 1122 and involved a disagreement over the leadership of the medieval church.
In the United States, there is a separation of church and state. While in more recent times this “doctrine” has been corrupted to mean that religious principles and organizations have no place in political/public debate, the original intent was to protect the church from being dominated by the state and to prevent any one church from gaining control over the government. Political officials do not appoint church officials, and vice versa. Despite cries from secular progressives today, the United States has never had an issue with church and state that remotely resembled the mingling of church and state in the Middle Ages—the kind of mingling the Founding Fathers were hoping to guard against.
In medieval Europe, however, religious and political power were thoroughly mingled. From the 400s, many church offices were appointed by the secular rulers or nominated by them and then approved or “rubber stamped” by church leadership. Even the pope was often nominated by the king or emperor. Secular authorities often intervened to solve church disputes. In many cases, bishops (who oversee a diocese, a large territory of many parishes) and abbots (who oversee a monastery) were appointed directly by secular rulers. These positions usually came with access to land and wealth, and many times the appointee was a younger son or relative of the secular ruler. Church offices were simply a favor that the ruler he could bestow or a position that he could sell to the highest bidder (one who would be loyal to him). The process of conferring these positions is called investiture.
In the 11th and 12th centuries, some church leaders as well as some reformers began to challenge the practice of investiture. Church leaders tried to assert more power, and secular rulers pushed back. Some of the nobles sided with the church, often for political reasons as they wanted the secular authorities who ruled over them to have less power. For about 50 years, there were armed conflicts between supporters of the pope and supporters of the Holy Roman Emperor. All of these quarrels and conflicts are known as the Investiture Controversy.
The controversy was finally settled in 1122 at the Concordant of Worms, where Emperor Henry V and Pope Calixtus II agreed that there would be a separation between religious and political authority and that the church should have authority to appoint church officials.
How did the Investiture Controversy impact Christianity?
As a result of the Investiture Controversy, the power of the church increased, at least in regard to making appointments to church offices, and the power of the Holy Roman Emperor declined. With a weakened emperor, local rulers were able to exercise more authority in their own realms. It was this fragmentation of power that, some 400 years later, helped the Reformation in Germany. Martin Luther was protected by Fredrick III of Saxony because the Holy Roman Emperor did not exercise ultimate control in Germany and was unable to enforce the punishment that the pope wanted to inflict on Luther. Because local rulers exercised increasing authority in their own realms, it was easier for parts of the Holy Roman Empire to embrace the Reformation without fear of retaliation from either pope or emperor.