I was talking to a co-worker who is Seventh Day Adventist and he mentioned the term Christendom. I asked him what he meant by that term and he said it was religions who claimed to follow Christ but do not. I asked him if this included JW's and he said it did as they were part of this false Christianity that Jesus would one day judge as unworthy. This is pretty much the same way the Witnesses use the term. Some years ago I heard another person use the term Christendom in a similar manner and it also included everyone but them.
This seems to be an easy term for the "we're the truth" religions to grab and lump everyone else in so when you make a comparison it's easy to show how united you are versus those in "Christendom". You say "we don't worship idols like Christendom" and yet most religions calling themselves Christian do not worship idols.
A Baptist could use this term and say "Folks in Christendom have said things like the Creator promised a peaceful and secure new world before the generation that saw the events of 1914 passes away, but we follow the Bible because Jesus said we do not know the day or the hour of his return."
Using the term Christendom to lump everyone else in is just an easy way out.
Donny