"Christendom" isn't really congruent with "Christianity". It was a medieval concept meaning "The collective society of the portion of the world where Christianity holds sway". It's kind of like how "The First World" is used to refer to the grouping of the U.S., Western Europe, and so on.
I think there are salient things to say about it as a historical concept that don't fall under the purview of the Christianity entry. -- Paul Drye
I had a vague recollection that they might not be exactly synonymous, but I reckoned that putting up a redirect from the non-existant link I found would at least make someone who knew better react. That seems to have succeeded. Now, the question is why you put the explanation here and not on the main page? --
Pinkunicorn
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