How was the Catholic Church corrupt in this period of the Renaissance?

Leaders of the Catholic Church during the Renaissance era certainly engaged in corrupt behaviors and acts. High ranking leaders of the church lived lavish lifestyles while they preached the holiness of a humble and modest life. Affairs, adultery, and pedophilic behaviors by church leaders were all too common.
One of the most notable examples of the Catholic Church during the Renaissance era was in their selling of indulgences. Indulgences were pieces of paper that church leaders would sell as an erasure of one's sins. The more indulgences one bought, the more one's soul was in good moral standing with God. Church leaders directly profited from selling indulgences. This corruption created a dynamic in which people with wealth were told they had more access to spiritual purity, and subsequently, Heaven, than poor people.
The Catholic Church was so powerful that it was essentially the law of the land is some areas of Western Europe in the 16th century. Anyone who defied the teachings of the church through presenting a different narrative were subject to be burnt at the stake.

The Catholic Church was quite corrupt in the Middle Ages and early Renaissance. Many popes looked at growing personally wealthy and powerful from their holy office and they did not speak much about spiritual matters. Priests and nuns who took orders of celibacy were sometimes promiscuous. There were even well-documented cases of popes having illegitimate children. In many towns the clergy were the most literate people, but some priests did not even know how to read, and they too looked to gain riches and power from their positions. While much of the clergy stayed true to its religious mission, there were enough members who didn't to provide satirists with plenty of material.
The Decameron takes place during the plague and it is a collection of tales told by young people who are leaving for the countryside in order to avoid the sickness. Since the Church was still the most important institution in Europe at the time, many of the tales in the book humorously describe the abuses of the Church.