Why was the Roman Catholic Church so powerful during the Middle Ages?
After the fall of Western Rome in 476 CE, Europe lacked a strong empire to unite the people. Instead, there were many smaller groups and tribes, such as the Franks, who ruled over their respective domains. One particular Frankish King, Clovis, wanted to unite several of the groups within the boundaries of modern day France. In order to do this, and in order to grow his military to the size necessary for conquering an empire, he allied himself with the only surviving institution in Western Europe from the fall of the Roman Empire—the Catholic Church.
Allying with the Church enabled Clovis to do two things. First, he used the economic power of the Church to grow his empire. Second, he used his alliance with the Church in order to convince other groups, who were loyal to Rome, to join his new Merovingian Empire (remember—Rome converted to Christianity in the 4th century under Emperor Constantine).
In the 8th century, the Franks and the Church faced a new threat from the South—the growth and expansion of the Islamic Umayyad Dynasty into Spain. At the Battle of Tours, 732, Charles "the Hammer" Martel defeated the Muslims in Spain, opening the doors for a Frankish Empire in Western Europe. This was realized by Charlemagne and the Holy Roman Empire.
The Holy Roman Empire, under the auspices of the Roman Catholic Church, conquered and united much of Western Europe. With this, the Church maintained power over much of medieval life.