Medusa is a monster in Greek mythology known for being associated in some way with the gods Poseidon and Athena. By Poseidon she conceived a pair of twin sons, namely the winged horse Pegasus and some other kind of creature called Chrysaor. Her relationship with Athena is somewhat complicated by the variety of its different versions. The most complicated aspect of her connection with this goddess, however, would appear to be a modern version thereof.
The Internet is replete with the assertion that Medusa was originally a beautiful young lady who once served as priestess of Athena before she was violated by Poseidon in the temple of this virgin goddess. I searched via Google for the source of this priestess back-story of Medusa, having myself taken it for granted that it must go back to at least one ancient mythographer.
Literally half of the top ten results from my search all simply and baldly state that Medusa was Athena's priestess but none citing its source for the statement. Two of them go as far as to say that she was the goddess's "chief priestess." Part of what makes Medusa's story tragic, as it is understood in this modern version, is that Poseidon rapes her and she gets punished for it by Athena changing her from a beautiful damsel into a most deadly thing of terror. More's the pity if this is how Athena treats her own cultic attendant.
As far as I've been able to find there is no Greek source which has any explicit mention of a story in which Medusa suffers a sexual assault. The Roman poet Ovid seems, in Book 4 of his Metamorphoses, to be first person to bring up such an idea, in which he says that Neptune (the Roman Poseidon) accosted Medusa in the shrine of Minerva (the Roman Athena). For some reason Minerva deems this event punishable by the transformation of Medusa's lovely hair into a mass of snakes.
No explanation is contained in the Metamorphoses, however, of how Neptune and Medusa find themselves in Minerva's sacred precinct, and the text nowhere says that Medusa was a priestess. According to Theoi.com, "Earlier Greek writers and artists... simply portray her [Medusa] as a monster born into a large family of monsters."
Apollodorus, writing around the same time as Ovid, or perhaps a bit later (in the 1st or 2nd century AD), is the only Greek source that seems to hint at the idea that Medusa's origin was perhaps different from her simply having been born as a monster. He says in Bibliotheka 2.4.3 that the slaying of Medusa by Perseus was Athena's own vanity project because "they say that the Gorgon was fain to match herself with the goddess even in beauty."
It isn't until centuries later, in the mediaeval Latin compilations called the Vatican Mythographers, that I find the next mention of the story of Neptune's violation of a beautiful Medusa in Minerva's shrine, and of Minerva thereafter making her into a snake-haired horror. Once again, though, the Vatican Mythographers simply reiterate the information found in Ovid and contain nothing about priesthood for Medusa.
No other source that I've found, Greek or Roman, ancient or mediaeval, mentions anything further about any sort of rivalry or relationship between Medusa and Athena/Minerva prior to the goddess becoming Perseus' patroness in his mission to slay the Gorgon. So where does this notion of Medusa being a priestess of Athena come from?
The closest to such a claim that I've come across is in the speculative anthropology presented by Robert Graves in his 1955 book The Greek Myths, on p. 17 of which he writes that:
A large part of Greek myth is politico-religious history... Perseus... probably, represented the patriarchal Hellenes who invaded Greece and Asia Minor early in the second millennium BC, and challenged the power of the Triple-goddess... Jane Harrison has pointed out (Prolegomena to the Study of Greek Religion, chapter V) that Medusa was once the goddess herself, hiding behind a prophylactic Gorgon mask: a hideous face intended to warn the profane against trespassing on her Mysteries. Perseus beheads Medusa: that is, the Hellenes overran the goddess's chief shrines, stripped her priestesses of their Gorgon masks, and took possession of the sacred horses—an early representation of the goddess with a Gorgon's head and a mare's body has been found in Boeotia... [T]he Hellenes annulled the ancient Medusan calendar, and replaced it with another.
But even in all of that Graves does not actually call Medusa a priestess, rather he says that this female monster was originally simply a mask of the great goddess whom he theorises was dethroned by later patriarchal usurpation in Europe.
Could this be the point from which, at present day, it has become such a basic assumption that Medusa's story starts off with her as Athena's priestess?
Or Conflation of Other Priestess Stories, Maybe?
Strangely, there are two priestesses of Athena who actually do occur in Greek myth and whose stories are less popular than that of this modern Medusa-priestess but which perhaps got conflated into the "neo-myth" in question.
The first priestess incidentally is connected with Medusa as well, her own name being Iodama. According to Pausanias, she once enters Athena's precinct by night, where, for some unexplained reason, Athena appears to her wearing a tunic with Medusa's face on it. Iodama, upon seeing that, turns to stone.
The other priestess, named Auge, is an Arcadian princess whose story appears in several versions. For some reason or another her father installs her as priestess in Athena's temple at Tegea, a position requiring her perpetual celibacy. When Heracles [Hercules] comes through the region, however, he impregnates her, she gives birth to his son Telephus, and mother and son thereafter go through a series of adventures both at home and abroad.
In no version does Athena ever punish Auge, unless one were to interpret the adventures she goes through and the threats to her life from her father, and certain other circumstances, as the work of the goddess. Instead the land in which Auge serves as Athena's priestess becomes barren as a result of the pollution of her having borne a child in the sacred precinct in which she then goes on to harbour the boy. Nonetheless in every version of the story both mother and child survive against nigh-impossible odds.
If the answer to my Question lies in none of the above, perhaps I've missed something in that one sentence of Ovid's in which Neptune happens upon Medusa. Could it have been understood that the only reason for the location of this encounter is that the temple must have been Medusa's workplace?