During the Middle Ages the Roman Catholic Church virtually controlled Western Europe. Guarding its position, anyone who dissented was labeled a heretic and burned at the stake. Having built its power on the ignorance of the peasantry, who were forced to pay tithes of 10% of their earnings or crops as they were taught that they would not able to get to heaven without the help of the Church, the essential doctrines of Catholicism were ignored as the Church instead concerned itself with the acquisition of wealth and power.
The wealthy were able to purchase high positions for their sons [simony] in the Catholic Church, positions that brought with them much political power and the assurance for their parents of a place in heaven. Along with the costs of baptisms, marriages, etc., the Church also raised revenues by the sale of indulgences, which some felt gave them a license to sin as they could later buy their redemption into grace, relics, and passage on pilgrimages.
By the fourteenth century, too, the doctrine of St. Augustine had been neglected terribly. That is, the doctrine that salvation was effected by a spiritual rebirth through good works and God's grace. Instead, salvation came about by external obedience and payment to the Church which did not always flow from Christian acts or religious faith.
From the years 1378-1417. there was a Papal Schism. With three simultaneous popes excommunicating each other, these popes brought much divisiveness and corruption to the Church. Other members of the clergy, too, were morally deficient, as well, as many priests came to their positions through political manipulations. They also knew little of the Bible of Catholic theology.
As evidence of the corruption of the Catholic Church during the fourteenth century, many of Geoffrey Chaucer's Canterbury Tales depict monks, priests, and nuns as avaricious, licentious, and generally more secular in their interests than religious.