The Roman Catholic Church was so powerful in medieval Europe as it was the center of social, religious, and intellectual life.
People were expected to attend church regularly, which meant daily at the time, which meant that churches and pubs were the center of a town's social life as it was where you saw your friends.
The Church also ran all of the schools and Universities, so a person educated in the medieval era was educated by the Church.
In short, the Church ran the show and owned a tremendous amount of land.