I'd go with the development of canon law.
Canon law has been in development from the earliest times of the church and continues into the present. But in the "Age of Faith" -- the medieval era during which the church was the dominant institution in society and culture -- canon law became highly detailed and affected many aspects of intertwined church and culture. And while Protestants later would object to much of what was being done in the Roman Catholic Church, their own structures and practices nevertheless were influenced by patterns learned from medieval canon law.
