Have you studied classical languages? It takes some explaining if you haven’t. Greeks in antiquity themselves found Thucydides a hard read and Pindar often barely intelligible.
One can more or less create a spoken language based on, say, Xenophon, but that won’t be sufficient for understanding Aeschylus’ lyrics only 50 years earlier.
To ‘know’ Greek is to know a changing language and literature across a thousand years or more and spread across the Aegean, Mediterranean, and Asia.
It’s not the same as learning a standard modern language unless, perhaps, you specify a circumscribed time and place, and the dialect in which it was spoken. And then there are disputable matters of idiom (the subject of philological scholarship) as well as meaning to contend with - even “just reading” a text requires that text to be accurately transmitted.