What was Christendom?
The term “Christendom” can refer to a few different things. In a sense, it can refer to all of the Christians in the world. In another sense, it can refer to a region where the majority of people are Christians. aka Catholics.
Another potential meaning would be in a country where Christianity dominates. Not necessarily a country where there were more Catholics than not, because a whole country’s laws and opinions can be swayed by the king’s point of view.
In its historical sense, the term usually refers to the medieval period in which the Christian world represented a significant power, comparing to the Muslim and Pagan worlds.