...Secularized. That's right...
Was I not the one to say just that?
...The kind of backward thinking that led to a term like "Christendom" being developed is in our rear view mirror...
The terminology, anyway, is, indeed, in our rear view mirror.
Only to be dusted-off and trotted-out for display on rare occasions, such as when discussing the domains of the world's various religions.
... This ain't Christendom.
In any discussion of the domains of the world's various religions, each of those domains has a macro-level name.
The domain of Buddhism.
The domain of Hinduism.
The domain of Islam.
The domain of Christianity.
The domain of Christianity is commonly referred to as Christendom.
Each domain relies upon both historical possession and present-day demographics to outline its boundaries.
With respect to Christianity...
If you live in Europe or the Russias or anywhere in the Western Hemisphere or European-speaking parts of Oceania or the Western Pacific, you live within the domains of Christianity... consequently, you live in Christendom... a macro-level domain reference that spans national borders and even hemispheric distinctions.
As an atheist, you might not be overly-fond of seeing the label being trotted-out and utilized from time to time, but c'est la vie, and your objections over an acceptable macro-level domain label have little bearing upon the correctness of its use or application.
It's not a common reference, nowadays, but it's an accurate one, based upon both history and demographics.
I'm content with that level of reinforcing argument.