Originally, Christendom was used as a term meaning 'the Christian World', so basically all the people who were Christian. It also referred to Christianity itself, such as in Norwegian where the word Kristendom means Christianity. As Christianity began to completely dominate the world, however, the meaning of it changed. By the mid-late Dark Ages, Christendom was used by the middle European powers to describe what they saw as the centre of the civilised world. Of course the "civilised world" could only include Christians. The following is a list of countries within Christendom during its various stages:

• Early Nations C.E. 600 - 1000 (Mid-Late Dark Ages)
  • Frankish Empire
  • Ireland
  • Mercia
  • Northumbria
  • Ostrogoth Empire
  • Papal States
  • Saxony
  • Scotland
  • Visigoth Empire
  • Wales
• Mid Nations A.D. 1000 - 1400 (Early-High Medieval Ages)
  • Aragon
  • Britain
  • Denmark
  • Castile
  • France
  • Holy Roman Empire
  • Hungary
  • Ireland
  • Lombardy
  • Papal States
  • Poland
  • Portugal
  • Scotland
  • Sicily
  • Switzerland
  • Valencia
  • Various Italian states¹
  • Various Spanish states¹
• Late Nations A.D. 1400 - 1700 (Late Medieval Age - Late Rennaisaince Age)
  • Britain
  • Denmark
  • France
  • Hungary
  • Italy
  • Papal States
  • Poland
  • Portugal
  • Scotland
  • Spain
  • Switzerland
  • Various Germanic states¹
  • Various Scandinavian states¹
  • Various Slavic states¹

¹This is not to suggest that they were solid states with defined borders as we know today, but rather masses of hotly contested lands where small warlords and dukes reigned.

After this the word Christendom, when referring to a specific area, wasn't really used. The colonies (i.e. America, Australia etc.) were never included in this term, and Christendom wasn't really related to imperialism. Eventually the word returned to its original meaning, "the Christian World", all those who are Christian. Since this time Christendom has been in decline, as the harsh enforcement of it has ended, and there is much more toleration of alternative religions.

Chris"ten*dom (?), n. [AS. cristendm; cristen a Christian + -dom.]

1.

The profession of faith in Christ by baptism; hence, the Christian religion, or the adoption of it.

Shak.

2.

The name received at baptism; or, more generally, any name or appelation.

Pretty, fond, adoptious christendoms. Shak.

3.

That portion of the world in which Christianity prevails, or which is governed under Christian institutions, in distinction from heathen or Mohammedan lands.

The Arian doctrine which then divided Christendom. Milton

A wide and still widening Christendom. Coleridge.

4.

The whole body of Christians.

Hooker.

© Webster 1913.