What kind of Greek does the GOARCH use? I'm trying to learn how to pronounce Greek in the Liturgy correctly and found that there several types of Greek (Ancient, Koine, Byzantine, and Modern Greek). Thanks.
From what I understand, it's a combination of Koine and Byzantine, but everything is pronounced like Modern.
"Itacization" -- the thing that the less phonologically aware British/American academics think of as "modern" pronunciation -- occurred in Koine Greek. Many Hellenistic papyri, especially from Egypt, show its influence. In other words, "modern" pronunciation was widespread before the time of Christ and has been dominate since the 2nd century AD. Check out Robert Browning's Medieval and Modern Greek
(Cambridge University Press, 1983) for the full story.
As for the language of the Liturgy: Anyone who can read Attic, Koine, or Byzantine would have no trouble understanding the Liturgy. In a practical sense, after 12 years as a reader of those three dialects, I don't find any significant differences among them (such things only mattered as a novice). Far more significant is the style, subject matter, and level of education of the author and reader. Same as English today. It's all "modern" English, but it's hard for someone with a 4th grade education to understand the prose of Saul Bellow or the poetry of T.S. Eliot; or a 10th grader to understand an academic article published in Comparative Literature
; or a college-educated humanities major to understand a study published in Cellular Oncology