OK, I’m no Orthodox, but I will play along. But I want something from you in return.
I am willing to delve into my largely obsolete library, to my
Complete Compact Oxford English Dictionary (OED). A fearsome set of two large, thick volumes which reduce the entire 16-volume OED into four microscopic-sized pages per page. The set comes with its own high quality
magnifying glass. Even WITH the magnifying glass and my own reading glasses, my 50-year-old eyes struggle to grasp the text.
Nobody uses the OED to look up the definitions of words (the usual purpose of a dictionary). The
singular purpose of the OED is to research the ORIGIN of words (regardless of language of origin). If a word is used in English, the OED will strive to provide the first use of that word, even if it was originally used in a non-English text.
I have Volume-1 of the Compact OED before me, turned to page 2012, which describes the origin of the word “Orthodox” (not just the plainly obvious Greek pairs, but when the word was (apparently) first used in publication as a proper noun, in any language).
I am willing to strain my aging eyes behind reading glasses and a magnifier to manually transcribe the origin of the word, which I believe will (finally) answer your question in the best scholarly sense available. But I ask you to do something first. I ask you to explain why the answer (whatever it is) actually matters.
If the OED cites first usage of “Orthodox” as a proper noun in the pre-Nicene era, or in the late middle ages, I want you to explain why it matters either way.
If you do that, then I will transcribe what I find in my OED and post it here. I don’t care whether I agree or disagree with your reason - I will hold up my end of the bargain either way. I just want to know why you think it matters.