If you don’t know about the Greek language question, look at the link: this won’t really make sense otherwise.
Neeraj Mathur asked in comments to Nick Nicholas’ answer to Who were the biggest enemies of Greek?
So in a sense, the Katharevousa partisans would have portrayed the Demotic advocates as the enemies of Greek heritage, while the other side would see them as the enemies of Greek folk culture. How heated was the actual debate?
Enough for people to be killed. 8 demonstrators in the Gospel riots in 1901 (protesting a translation of the Gospel of Matthew into the vernacular), and 2 demonstrators in the Orestes riots (Ορεστειακά) of 1903 (protesting the translation of Aeschylus’ Oresteia into the vernacular).
Of course, as with any such organised violent conflict, other stuff was going on; Greece had been militarily humiliated in 1897 by the Ottomans, Greece was panicked about Bulgarian encroachment to its north, and the Gospel translation was sponsored by Queen Olga, who was Russian. So, Hey Presto, moral panic: “the Russians are undermining our religion to turn us into Slavs”, combined somehow with “the Protestants are out to deracinate us” (since the translator lived in Liverpool, and there had been Protestant missionary activity in Greece for decades).
Now, this is not how a Diglossia is supposed to work. In normal diglossias, like you get in Egypt or Haiti or, for that matter, Greek in Cyprus, you have a Low variant and a High variant, people know when to use which, and it’s just the way things are done. That’s what Greece was like up to 1880. (Not the Ionian Islands though, which used the vernacular in literature, and were not in Greece until 1864.)
And the ideology was pretty universally respected: Puristic Greek would efface the orientalist shame of the vernacular, and restore Greek to something more respectable, though not as a full revival of Ancient Greek. (That made Puristic a quite unstable compromise, varying by author, and adrift between Koine and Mediaeval Greek.)
That started breaking down in the 1870s, with Valaoritis’ vernacular patriotic poetry being given official recognition. It blew up with activists in the 1880 and 1890s, of whom Psichari was only the loudest. And the dispute then was ideological, as Neeraj guessed: “enemies of our Hellenic heritage” vs “enemies of our Romaic heritage”.
Add to that though that Psichari was a Neogrammarian: the somersaults that Puristic Greek had to do to compromise between Ancient Greek and the vernacular offended him as a linguist, and he advocated a linguistically consistent morphology and phonology.
If Psichari sounds ridiculously folksy to modern speakers, it’s not because he was linguistically wrong: Standard Greek phonetics is utterly ridiculous because of its spelling pronunciations of ancient Greek. It’s because Psichari was sociolinguistically clueless (not helped by the fact that he did not live in Greece). The next generation of activists, such as Triantafyllidis and Tzartzanos, were more sociolinguistically aware, and advocated a vernacular closer to what is used now, with more concessions towards Puristic.
By their time, Demotic was universally used in literature; and the Greek diglossia was derailed: it was now a competition between two norms, Puristic and Demotic, for the status of High language. And with Demotic universal in literature, Puristic was on the back foot—though it remained universal in government and the church).
In Psichari’s generation, the conflict was Hellenic vs Romaic, but it was not yet Left vs Right. Psichari himself was a royalist; the early Communist Party dismissed the Language Question as a bourgeois distraction. And though “enemies of our Hellenic heritage” nowadays sounds reactionary, at the time it was introduced in the 1810s, Puristic was actually a vehicle of the Enlightenment, and seen as progressive.
By the 1920s, though, Puristic vs Demotic had settled into Right Wing vs Left Wing. People could work out your political persuasion in Greece, by whether you used the 1st declension or the 3rd declension in your genitives of –is nouns.
No, I am not exaggerating: I lived in Greece at the very tail end of that language feeling, and an -εως genitive still makes me wince. It’s one of the many conflicts Dimitra Triantafyllidou and I delight in having. And hey, it’s better than Turkey, where the political shibboleth was the shape of your moustache.
What killed Puristic in the end was the 1967 dictatorship’s reactionary enthusiasm for it: when democracy was restored, Puristic was dispensed with in government with universal revulsion. What replaced it of course was not Psichari’s ideal; the Constitution even warns that Demotic shall be adopted “without extremist features”. And the flavours of Standard Greek that have prevailed have waxed and waned in their archaisms in the decades since. But Greek has mostly settled down into normal registers, rather than street fighting conflict.