The Catholic Church's power in the Middle Ages was primarily derived from a combination of belief, money and illiteracy. That is, most people in that age strongly believed in God, heaven and hell, and the Catholic Church was the center of that belief.
The Catholic Church put forth the belief that people could only get to heaven through the Church. Most people in the time period donated 10 percent of their income to the Church, giving it incredible wealth, and the Church did not pay taxes. On top of regular tithing, parishioners often had to pay for ceremonies, such as baptisms, and they paid penances also. Others gave land to the Church, meaning it held about 30 percent of Western Europe in land. The other part of the Church's power came from the fact that most parishioners were illiterate, which meant they had to rely on the Church to interpret the biblical texts.