There is a popularized definition of “myth” that the term refers to “other people’s religion.” In other words, THEY have silly beliefs, while my beliefs are perfectly sensible.

In an ancient setting, myths are the recorded stories drawn more or less faithfully from contemporary oral tradition. These included legends (stories told to be believed) about a wide variety of things including to explain the origin of things (etiological legends about primordial times) or events in a more recent past (historical legends).

In the expanse of Greek history – to use your example – there were likely always skeptics, but there was also appears to have been a tendency for people in more urbanized, later settings to see the stories recorded in their texts as having meaning that reached beyond the “literal truth” of the story. Consider how modern people react to the story of Noah and the flood – some believe it to be literally true while others react to the story as an allegory that describes humanity’s relationship with God. Still others simply scoff at it and move on. This was likely happening in a place like Athens during the time of the great philosophers, for example.

Religions do automatically evolve into myths. People interact with their own religious stories – and those of others – in various ways. Consider the difference separating “the Resurrection story” from “the Resurrection myth.” As long as there are believers, the story will remain a story – something to be taken as true as a matter of faith. As long as there are people who do not believe, the Resurrection story will be regarded as a myth. But to call it a myth is to be derisive and hurtful to believers, so wielding the term “myth” needs to be understood as a matter of weaponizing language.

Does that sort of weaponizing occur now and will it occur in the future? Of course, but that doesn’t mean that “religion has turned into myth.” “Myth” is in the eye of the beholder. That doesn’t really happen until a religion is completely extinguished and people can look back on it objectively.

Because of the derisive nature of the term “myth,” I always told my folklore students to avoid using it except when discussing long-dead belief systems and their oral traditions. The following is an excerpt from my Introduction to Folklore that I provided to my students when I taught the introductory course at university:

Something also needs to be said here about myth. People use this term awkwardly. In a European context, myths tend to be the artificial constructs of ancient and Classical-era priests or literate people who sought to weave folk traditions into a comprehensive whole. The exercise often had political purposes, designed to provide diverse people with a single set of beliefs and stories. By reconciling similar traditions, the shared culture of these groups could be seen as more important than the differences, justifying the central rule of the king and his priests. Myth is also a way of organizing and reconciling folk traditions, which by their nature can be contradictory and highly localized. Myth tends, however, to make gods of supernatural beings, giving those powerful entities a status – for modern readers – similar to the Judeo-Christian-Islamic God, even when this comparison is not justified. Of course, it is also important to point out that myths were stories that were told – and then written down – and they were different from religion itself. Many myths were simply the shared cultural inheritance of a group of people.

In general, the word myth is best set aside when discussing more recent folk traditions, recognizing its proper status as a literary genre. Nonetheless, ancient documents recording myths can assist in understanding the history of various stories and beliefs. The authors of these texts were, after all, the first folklorists, and they were the only ones coming close to practicing the craft at the time.

Some folklorists carelessly use the term myth to denote those legends that deal with a fantastic, remote time. This primal era saw the creation of many familiar things such as day and night, fire, animals, people, mountains, and all other aspects of the present world. Folklorists properly refer to these stories as etiological legends explaining the origin of things. Sometimes, however, people interchange etiological legends with the word myth. The problem with this is that “myth” can imply something that is inherently wrong, linked to “primitive” superstitious beliefs. When the term “myth” is used for the folklore of existing cultures or for the traditions that were viable only a generation or more ago, it can take on an insulting, derogatory tone. It is best to reserve the word “myth” for ancient and Classical-era texts.