To get specific: let's say I'm a rather well-educated person living in England in the 15'th century and an equally well-educated friend of mine is doing something reckless. I say something like "be careful you don't fly too close to the sun." Does my friend know what I'm referencing?
Or I'm watching two knights fighting and I compare them to Achilles and Hector. Or I'm setting out on a journey at sea to find something, and I compare myself to Jason and my companions to the Argonauts. Or maybe Odysseus. Are these things I would know about?
If so, would I have read them from books, or were people maybe telling the stories orally? In the Canterbury Tales, The Knight tells a story featuring Theseus and Hippolyta- does this mean that Greek/Roman Mythology was still being transmitted orally and could be referenced and understood by different classes of people?
A rather well-educated person (which implied a knowledge of Latin) in England and elsewhere in the Late Middle Ages (and the early MA as well), would have a good grasp of classical mythology courtesy of the preservation of the works of Latin authors (esp. Ovid’s Metamorphoses--the greatest compendium of Greek and Roman myths, Virgil’s Aeneid, the poems of Horace, etc.) and the works of the early medieval authors who compiled classical stories from their sources. For example, the Mythologies of Fulgentius (d. early 6th c) explicitly set out to catalogue Greek and Roman mythology, though its purpose was to legitimize them by finding Christian allegories in them. The Etymologies of Isidore of Seville (d. 636), widely known throughout the MA, was an encyclopedia which didn’t focus exclusively on mythology but which assembled the most important classical myths and, like Fulgentius, gave them a Christian twist. Augustine’s well-known City of God was likewise not meant as a collection of myths, but in an effort to refute “pagan” gods, nevertheless told their stories. Medieval giants (well beyond “rather well educated,” of course!) like Dante (who knew classical mythology inside out), Boccaccio (ditto), Petrarch (proto-Renaissance), and Chaucer all knew basics of classical mythology. The early 14th-century Ovide moralisé (Moralized Ovid) was a complete translation of Metamorphoses into Old French, which was translated into English in the 1480, giving our educated person another way into Greco-Roman mythology if their Latin was weak or absent. It’s worth noting that even a “rather well-educated person” in the 15th century would not know Greek, which was a language just being taught again. Source: The Classical Tradition, ed. Grafton, Most, & Settis (2010) pieces together most of this. “Oxford Bibliographies" online has a big list of relevant books at Classics in the Middle Ages