In the fifth century, the Roman Catholic Church filled the void in power caused by the collapse of the Roman Empire. In the place of the Roman emperor, the pope became the new religious and political authority in Western Europe. The power of the church rested in its status as the gatekeeper of heaven. Across the spectrum from kings to peasants, people were terrified of being denied access to paradise.
The church consolidated its power through economic dominance. Peasants were required to labor for the church for free during a portion of their working week. Additionally, everyone rich and poor had to tithe ten percent of their income to the church, but the church was free from taxation. Without baptism, people couldn't go to heaven, and they had to pay to be baptized. The church also made massive amounts of money through the sale of indulgences, which gave absolution from sins.
Because of its immense wealth, the church was able to accumulate vast tracts of land and was able to construct immense cathedrals. People looked on these architectural marvels as symbols of the power of the church.
Scholars in monasteries and convents were also the primary custodians of books and of education. Before the invention of the printing press, books were handwritten one by one, often with ornate embellishments. The wealthy often sent sons and daughters to monasteries and convents as signs of their devotion to God. After taking vows of poverty, chastity, and obedience, these formerly privileged people would minister to the poor, the old, and the sick, making these sectors of society dependent upon and indebted to the church.
The church also held ultimate political power during the Middle Ages, because kings were as fearful of being denied access to heaven as commoners. It was the pope that crowned Charlemagne the Holy Roman Emperor. The pope also authorized and blessed the Norman invasion of England. Popes incited the Crusades by offering eternal salvation to warriors who would fight to reclaim Palestine from the Muslims. The power of kings and queens depended upon their allegiance to the church.
We see, then, that the lure of salvation and threat of damnation proffered by the Roman Catholic Church gave it immense religious, economical, and political power during the Middle Ages.