Christianity has been a very influential religion in the development of western civilization. During the middle ages the Catholic church had pretty high authority and influence, even to the point that it had authority over the nobles and monarchs. But what was the idea of Christendom, what does it mean?
Christendom in short is the international society of Christians. The reasons why Christendom was an international society was because monasteries especially during the middle ages would send monks from region to region. Nationality was never an issue from the Church, and was never considered. Many Abbots and Bishops governed regions which were very far away from where they were originally from. Another example of nationality not being an issue for Christendom was in the example of university professors during the middle ages.
The church during the middle ages also played a role in peacemaking. An example of this is with the figure Pope Gregory VII, who played a role as an arbitrator between King Philip I of France and William the conqueror. Also the church was capable at the time with enforcing negative sanctions to even nobles and monarchs, for example in the form of excommunication.
In conclusion, Christendom is the international society of Christians. Monks would go from region to region, and other church officials would govern areas far from their origins, so nationality was never really considered in Christendom. Also the church played roles as a peacemaker and was capable of enforcing negative sanctions such as excommunication to anyone even individuals with high power and status.