You got my brain hurting late at night.
In English there is also an opposition between "Christendom" and the "Muslim world", especially if we are talking about the crusades. I guess that's clear. And to talk about "a Jew living in Christendom" makes sense to my ears at least.
So does that mean the English word shares that sense with the French word, but also has another sense ("the whole body of Christians") which the French word lacks? (Apart from among French Witnesses who adopt the English sense)
In English Witnesses are fond of talking about the "missionaries of Christendom" and their bad deeds. This is to avoid saying: "Christian missionaries". What does a French Witness use there? Is it the same word French people would use generally in connection with missionaries?
I think this is where German Witnesses also run into difficulty between Christenheit and Christentum. My wife is asleep so I might be totally wrong about that too, but I am sure she said that to Witnesses Christenheit is the worst of the two, and the one they use for cross-wielding missionaries and so on.
French and other non-English speaking Witnesses should not feel too hard done by because as I say, what the Witnesses already do to their native English in the literature is pretty inventive also at times.