How did the beliefs of the Church affect Medieval European society?
The beliefs of the Church had a huge impact Medieval European society-- in fact, the entire structure of society was based on Church teachings! The Church was really the only remaining intellectual institution after the fall of the Roman Empire in Western Europe, so what they said went! The Church taught (and people believed) that God had dictated that there were three natural classes (or "Orders") of society. These were the Oratores ("those who pray," the Church,) Laboratores ("those who work," the peasantry,) and Bellatores ("those who fight," the nobility.) Each Order had their role in society. Primarily, the peasantry lived and worked to support the nobility, and the Church both reinforced this idea but existed somewhat outside the parasitic relationship between the other two classes. Becoming a member of clergy (as in a nun, priest, or monk) was really the only way for a peasant laborer to escape their weary lifestyle.
This structure of three Orders laid the foundation for and developed into the Feudal System. It still maintained the three general classes of society, but was far more complex in terms of who owed what to whom. Of course, the Church also played a role in the more intricate beliefs and actions of people's lives. The scholar Burchard of Worms even wrote a text on how to repent for some very specific sins! Everything in Medieval society, from the class you were born into, down to the sinful nature of eating scabs-- yuck!-- was influenced by the Church.