What exactly does the WTS mean by "Christendom"? Are these the nations where Christianity is the dominant or national religion? But aren't these nations members of the UN?
As far back as I can recall the WT (JW's) have always referred to Christendom as all false religions that are claiming to follow Christ.
It is the predominant part of Babylon the Great (all false religion, the Great Harlot).
JW's think of themselves as the only ones representing true Christianity.
So the UN (Image of the Beast) is all the nations it represents finally turning on the Harlot(Babylon the Great) at some point.