In 1040 the Church forbade the barons to make any attack on each other between sunset on Wednesday and sunrise on the following Monday, or upon any ecclesiastical fast or feast day ... on pain of excommunication.

This excerpt illustrates what?

The excerpt shows the extent of power the church in medieval Europe have over the people.

Explanation:

During Medieval times, since most of the European nations are Christians, the church holds the highest power, be it religiously or politically. Everything had to be done with the consent of the Church. From this excerpt, it is evident that the church can impose any restriction on it's congregation and the whole society as a whole whenever it wants and however it wants. By forbidding any attack "between sunset on Wednesday and sunrise on the following Monday, or upon any ecclesiastical fast or feast day " shows just how powerful the church is, capable of making rules in any specific day or time of the day.