What were the Christendom actually?

Answer 1

Christendom was all the states of the West that practiced the Christian religion. During the Middle Ages, people mostly identified themselves with their religion - Roman Catholic - as opposed to their nationality. The idea of a Nation State was not yet en vogue. Many disputes were arbitrated by the Church and all crowned heads of state were blessed by the church, an act which gave them legitimacy.

When rulers like Henry VIII decided that they could rule by divine right on their own, the unity of Christendom under the one church was broken forever. Henry created the first national protestant state and from thence on Englishmen considered themselves English and Protestant before any other idea of identity. Many other nations of Northern Europe followed suite by becoming Protestant and rejecting the churches authority. This rejection was, for all purposes, a declaration of independence and many Kings and Princes followed the Protestant path for less then wholly religious reasons. The church could no longer claim any wealth or taxes and their lands and treasures were appropriated by the state, enriching King and Noble alike.

The Popes were not happy and declared counter reformation. This succeeded in so much that it stopped Protestantism as a nationalist movement from spreading outside of Northwestern Europe.

Answer 2

Christendom was, in the Middle Ages, the Christian world, primarily the European countries and the Byzantine Empire--basically the modern countries of Portugal, the portions of Spain that had been wrested from the Moors, France, England, Scotland, Ireland, Norway, Sweden, Denmark, Germany, Austria, Italy, Hungary, Russia, Poland, Greece, the parts of Turkey that were part of the Byzantine Empire, the Czech Republic, Slovakia, and that's an extremely short list.

Common usage, though, generally limits 'Christendom' to Western Europe.