That “church” never is used in the OT is a rather simplistic, and pretty irrelevant, statement. I would not expect the word church to be used in the Old Testament, just as I would not expect to see paraclete, Christ, or apostle
Correct… temple is the word for the OT, The point again is that it is Jesus Who introduces the word as something He is going to build ONE of. It is Jesus who also introduces the paraclete, the comforter.
Can’t really disagree here.
Now we get to the crux of the matter. Define “church.” In it’s simplest form, it means “those called out (of the world)” or simply “assembly.” You are assigning a non-Biblical definition to the word.
Let’s, for the sake of discussion, use your definition. Again it is Jesus Who is doing the calling into His Church. Not Luther, not Calvin, not Smith, not any man or woman. And since it is His, He is the one Who makes all rules, is the source of authority, and introduces the sacraments, etc.
I never mentioned apostasy, nor do I personally consider Catholics, or most other Christians, apostate.
I was not accusing you of that. I apologize if you made that mistake.
The problem is that “other Christians” often justify their (non)church by saying that their beliefs are the early beliefs which the Catholic Church abandoned.
You can repeat it as many times as you like, but that doesn’t make it true.

And I can remain silent… and it is still true.
The Orthodox have a rather convincing counter- (or perhaps we should say, parallel) claim. Who’s right?
Both. One is referred to as the Eastern, one the Western Church. Both have valid orders, valid sacraments, valid liturgy. One has continued the office of Vicar of Christ, one has abandoned that gift(temporarily)