In case you wonder why 2 everyday use words changed: Οἶνος & ἄρτος are words that were "forbidden" for daily use by the orthodox church and are used solely by the clergy for the "blood and flesh" of Jesus. So "άρτος" in modern greek is the sacramental bread only and "οίνος" the sacramental wine. Κρασί comes from medival greek κρασίν < ancient greek κρᾶσις οίνου (mixture of wine) and ψωμί from medival greek ψωμίν < ψωμίον (small part)< ancient greek ψωμός <ψώω (grind, cut in small parts). Οίνος & άρτος are still used in colloquial greek as synthetics: οινοποιείο=winery, αρτοποιείο=bakery, οινόπνευμα=alcohol, αρτοσκευάσματα= baked goods etc.
It was not forbidden with a decree or law, that's why I put it in quotation marks, sorry if I expressed it wrong. The state and people were religious to the core then and it was considered a blasphemy to utter sacred words without a specific religious reason (in the same notion, it was a blasphemy to directly address god: δόξα σοι ο θεός: glory to you, god, with god being in the nominative instead of the vocative). Two of the words tied closely to god was οίνος άρτος (the blood and flesh) so new words were invented for those things, so that people did not refer to god aimlessly. That's what I was told in school, from my Byzantine history teacher, at least. ;)
I don't know why ended up in this theological discussion here, but to address troll1995's original question about using the nominative case of a noun to address someone, (for all practical purposes) the answer is NO. To be grammatically correct you need to use the vocative case of the noun/name (i.e. 'ανθρωπε, Αλέξανδρε, Χαράλαμπε), BUT if the name/noun is less than three syllables you use a modified vocative case for colloquial Greek: (i.e. Γιώργο, Θάνο, κλπ.). However, I don't think this discussion belongs here; maybe the course masters can move it to the proper place...
It is a defective phrase sentence. The full form of it would be Δόξα σοι ος ει ο Θεός -> "Glory to you (who are) the God". As you can see, there is nothing to do with preventing blasphemy. There are numerous other phrases, ecclesiastical and colloquial using vocative for God.
Was there any reason that they were saying "Glory to you (who are) the God", and addressing him in third person? The fact that is defective does not change the fact it did not address him directly. And so do any Byzantine chant i know of. There are noumerous phrases using vocative for God really, but not from back then. (Άγιε, Κύριε was used, but never Θεέ)
Native Greek speaker here (who lives in the US): I respectfully disagree with Duolingo! For people who learn Greek it is good to know that the word γυναίκα is used to refer to "wife" in addition to its primary translation of "woman", but it needs a possessive pronoun i.e. η γυναίκα μου or η γυναίκα σου, for "my wife/your wife". If I want to translate the colloquial expression "the wife drinks wine" (using the article and not a possessive pronoun), I would say η σύζυγος πίνει κρασί.