What was Christendom?
Christendom was a time when, even though the world is divided up into countries, a Christian was part of one society. It was a union of all monasteries, church buildings, and Christian organizations. Monks, abbots, and bishops sometimes governed locations far from their countries and places of birth. A worldwide unity of the above-mentioned subjects was Christendom. The primary and overwhelming majority of “Christians” were catholic.
Here is an excerpt from the Wikipedia article:
…The pontificate of Innocent III is considered the height of temporal power of the papacy. The Corpus Christianum described the then current notion of the community of all Christians united under the Roman Catholic Church. The community was to be guided by Christian values in its politics, economics and social life. Its legal basis was the corpus iuris canonica (body of canon law).
There isn’t much else to describe Christendom as other than a uniform community Christian people, art and architecture, religious leaders, and literature.