Where and what is Christendom and why was the concept needed?

Answer: The English language has two related words, Christianity and Christendom. In English, both terms are either used in the sense of Christianity, or Christendom refers specifically to those parts of the world in which Christianity dominates. The German language knows the two terms Christentum and Christenheit. Christentum corresponds more or less with the English Christianity, i.e. the religion which is derived from Jesus Christ, based on the Bible and which is split into the three large branches of the Orthodox, the Roman Catholic and the Protestant Christians. The term Christenheit is not an exact synonym of the English term Christendom, but it is very similar. It refers to the totality of groups of people and institutions which are dedicated to the Christian faith, or which have been largely defined by it. In the narrower sense, the term Christendom refers to the reality of the Christian dominated world of the medieval period, i.e. the period in European history starting with King Constantine. The Christian medieval period, i.e. the epoch of Christendom, was marked by a close relationship between secular government (lat. imperium) and the hierarchically structured church (lat. sacerdotium). However, even in the periods in which the two powers were most closely linked, they were still clearly seen as different from each other.