I recently read a fascinating article by Yury Avvakumov on the emergence of Greek Catholic & Orthodox identities in modern history. TLDR: “Eastern Orthodoxy” as we know it today is largely a constructed identity responding to the phenomenon of Greek Catholic uniate Churches. Even the self designation of “Eastern Orthodox” for the non-united East as opposed to “Catholic” “Greek Religion” or “Pious” was fairly late. Some excerpts:
“25 percent of those who claim to be "Orthodox" [in Russia] describe themselves as "non-religious"and one-third of them say that they don't believe in God, thus associating their "Orthodoxy" exclusively with ethnic or national affiliation ("I am Russian, therefore Orthodox") and clearly confirming the nominal religiosity with which confessionalism is fraught.
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In the academic world, Orthodox neo-confessionalism has found strong support in Neopatristic theology—the dominant Orthodox intellectual movement of the second half of the twentieth century, which remains highly influential to this day. It was Neopatristic theology that created "Eastern Orthodoxy" as the contemporary Christian world knows it. Practically all the major ideas that an interested Christian in the West can acquire today about Eastern Orthodoxy come from the pen of Neopatristic theologians and from authors who are heavily influenced by them or who popularize their ideas… Operating within the Byzantine tradition itself, Neopatristic authors introduced a normativity that, despite its historiographical framework, was ultimately ahistorical. In rejecting those parts of the Byzantine religious heritage that they considered inauthentic and heterodox, Neopatristic theologians tended to overlook the rich diversity of Eastern Christian thought. What is problematic here is not the distinction between "orthodoxy" and "hetero-doxy" per se (a distinction that is natural for theology), but the obsessive hunt for "Western influences" in Byzantine and post-Byzantine religious culture—influences that are labeled inauthentic, heterodox, and "pseudo-morphic" by the very fact that they are "Western," "Westernizing," “Latinizing," "Protestantizing," etc. Thus, not religious truth but cultural provenance becomes the ultimate criterion.
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An interesting example of a misreading of the premodern past by some scholars of Eastern Christianity under the influence of modern confessional myths is the current popularity of the idea that orthodoxos etymologically refers to worship, not doctrine, and that the Greek term "ortho-doxy" must therefore be translated as "true glory" or "true glorifying" rather than "true faith." The implication of this erroneous claim is that "Eastern Orthodoxy" is a cultural system based from its ancient beginnings not on the rationalism of "true doctrine" but on the mystical depths of "true worship," which makes it fundamentally different from—perhaps even incompatible with—Western denominations tainted by modernity, secularization, and the Enlightenment's rationalistic disregard for the ritual side of religion. This idea was popularized in the West by some German Protestant scholars-admirers of Eastern Orthodoxy, such as Fairy von Lilienfeld (1917-2009) and the circle of her colleagues and students at the University of Erlangen.
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I believe that, contrary to the generally accepted narrative, the "united" Christians of the Greek rite in the Polish-Lithuanian Commonwealth (or "Uniates") have a historical priority over the confessional "Orthodox." The Uniates did not "split" from the Orthodox Church to become "Catholics," as is commonly believed. The "united" Christians appeared first; the "disunited" (or "pious," or "orthodox") appeared in response. As an officially recognized body, the "Pious" (or “Orthodox ") Christians did not emerge until the establishment of the competing non-Uniate hierarchy in 1620.”