An Introduction: Mythology and Religion
At the crux of this inquiry, we have the question of what the difference is between mythology and religion; as such, before we can delve into the particular usages, it might behoove us to determine what these terms mean.
A myth is merely a traditional story, such that is meant to explain or shed light on the (especially early) history of a people or place, or to explain phenomenon whether natural or social.
The veracity of such stories is not an element of the term; as such,
just because something is mythology does not inherently preclude that thing from being true.
Take the Big Bang, evolution, and that whole facet of scientific and historic thought. We might look at various pieces of evidence and say, "yup, no myth here. This is clearly simple evidence," but once we fashion it into a story, take that evidence and try to explain origins and early history, we mythologize it -- because that's what myth is. The ancients took what they saw and knew, and they fashioned that into stories to try to explain the world around them, and we call that mythology.
You can see this concept in play with J.R.R. Tolkien's concept of the "True Myth," his designation for the stories behind Christianity, which he considered the different mythological systems reflect; this belief can be seen in how he constructed his own Legendarium. Given the influence on this exerted by his particular beliefs, one might even suggest this mythopoeia was a religious expression.
Because
religion, on the other hand, stems from a particular belief (some would say in a god or gods, specifically, but that seems a bit narrow), which then takes form in people's lives -- worship, or the actual practice of the religion --; the key here is that religion is put into practice and takes form, in some way, in the lives of its adherents.
For these purposes, myths are these sorts of stories, regardless of the truth behind them; religion is the manifestation of those stories in people's lives. The myth and the religion are separate things, though they may be intertwined.
Where Myth and Religion Separate: Examples
Paranoodle referenced earlier the Mayan religion, which can be, actually, a good example. One might most often hear in this area of the stories, the gods and characters and occurrences, the mythology, as it were; once we begin talking of priests or rituals or customs or other manners in which cultural beliefs take form in actual practice,
now we've shifted from Mayan mythology to Mayan religion.
It's the same with the Greek example offered in the initial question: when we consider the stories, we examine Greek mythology; when we study the associated practices, we are looking at Hellenic religion. A Hellenic Reconstructionist, for example, will
look at the mythology in order to reconstruct and practice the religion.
For an easy example from what is referred to in the opening post as religion, we can see this reflected in the recorded words of Jesus, who did not say that "pure and undefiled religion before God the Father is this: that the world was created in seven days and all was good before mankind fell, that the world was covered in a flood as judgement, that the Hebrew people continued to turn from the Lord and continually fell into captivity even as their God always recieved them back, and so forth," but that this pure religion is "to look after orphans and widows in their distress and to keep oneself unstained from the world."
Religion relies on the manner in which one's beliefs manifest, not the stories that might be basis for that religion.
In Answer to Your Question
The above examples provide an explanatory framework for why we might think one thing is considered myth and another religion.
The closer we are in proximity or knowledge of people practicing a set of beliefs, the more likely we are to speak of it in such manner. We speak regarding these groups with their religious expression, not necessarily of the underlying stories specifically.
The further we are, however, in proximity or knowledge of people practicing a set of beliefs, the more likely we are to speak of it in
that manner, regarding not the people and their religious expression, but regarding the mythology, the underlying stories behind it.
Consider the example above, where we speak of Mayan or Hellenic stories, characters and events, of the mythology, but when we speak of customs and rituals and what the people
did, it's of their religion.
Whether we're speaking of the stories or of how those directed people's lives is what determines whether we're speaking of mythology or of religion, and different people in different cases are more likely to refer to one or the other.
So wherein lies the contention?
Now, this all gets muddied when different meanings of the word "myth" are conflated in common parlance, whether intentionally or through ignorance. As we've looked at these stories and declared them false, the word "myth" has taken on an additional meaning, simply that of something which is untrue.
We've already determined that, for the area of discussion proposed by the initial question here, myth and mythology do not necessarily take on this meaning. But some do not understand this and, as such, are wont to take offense at some of these stories being referred to as such.
This is compounded by certain factions, who, regardless of whether they understand the term (and I tend to assume they don't), weaponize it as an attack against beliefs with which they disagree, saying "that's only a myth."
And this can feed further into the first point, while also denigrating the basic idea of mythology as less-than.
These points likely play into the question at hand, but, nevertheless, they ignore the basic meanings and distinctions.
convention?
