The Collaborative International Dictionary of English v.0.48:

Christendom \Chris"ten*dom\, n. [AS. cristend[=o]m; cristen a
   Christian + -dom.]
   1. The profession of faith in Christ by baptism; hence, the
      Christian religion, or the adoption of it. [Obs.] --Shak.
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   2. The name received at baptism; or, more generally, any name
      or appelation. [Obs.]
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            Pretty, fond, adoptious christendoms. --Shak.
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   3. That portion of the world in which Christianity prevails,
      or which is governed under Christian institutions, in
      distinction from heathen or Mohammedan lands.
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            The Arian doctrine which then divided Christendom.
                                                  --Milton
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            A wide and still widening Christendom. --Coleridge.
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   4. The whole body of Christians. --Hooker.
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WordNet (r) 3.0 (2006):

Christendom
    n 1: the collective body of Christians throughout the world and
         history (found predominantly in Europe and the Americas and
         Australia); "for a thousand years the Roman Catholic Church
         was the principal church of Christendom" [syn:
         Christendom, Christianity]