It's difficult to identify the original purpose and it actually changes
I'll try to make the argument for the frail limits between mythology, fiction and religion, and the need to use common sense in judging cases and why most mythology should land on the fiction side. Jason Baker and Radhil both argue that the intended purpose of the original author is what sets them apart. I think many stories lack an original author and are retold and rewritten over time with different purposes.
Let's take the example of Norse mythology. Most things we know about comes from the Eddas, two books compiled in the 13th century by Snorri Sturluson (among other anonymous and proposed authors). At least the Prose Edda came to be several hundred years after Scandinavia had been Christianized and Snorri wasn't a believer. Maybe his purpose was that of a scholar, maybe he just wanted to entertain, but he certainly wasn't writing a religious text. We can have similar doubts about Virgil's intention when writing the Aeneid. He's considered one of the fathers of Western literature.
On the other hand we have works considered fiction when published that inspire very religious feelings. The Jedi religion is one of them, that you may consider extreme, but you should know there are others like Cao Dai that considers Victor Hugo's works sacred text. We have science fiction authors, like Hubbard, whose works seemed originally pure science fiction, and the forming of a religion changed some of those works status.
What is a religious text sometimes needs to be decided in the site
It's not my intend to hurt religious feelings, I've none of my own but I practice and expect tolerance for what others believe. But that being said, I don't think that what any group claims to be a religious text is to be considered one for the sake of this site. I will quote some of the comments of the policy thread that represent how I feel:
Valorum: For the record, the Greek, Roman religions plays are fair game for being described as religious myths, largely because all of their followers died out centuries ago. The Epic of Gilgamesh, by comparison was written as a work of fiction.
Thoth19: That is false. Apparently Panagiotis Marinis still worships Zeus. The point you should be making is that these minorities are small enough that they likely won't cause a political controversy and if they do the negative utils will be small compared to the utils of the ease of utilizing SE for everyone else while offending them. Claiming that people don't exist is a poor mechanism because proving a negative is hard and there are many people.
Valorum: Also, the fact that a single individual has a non-standard religious belief doesn't give him the right to dictate SE policy on religions, any more than the occasional 7th Day Voldemortian would...
We can use consensus and common sense as guidelines
Where do we draw the line? It's really thin. Sometimes we don't know the original purpose or it changes with time. I think there is always a level of judging when deciding that an example is or isn't a religion. Sometimes it's easy (the Bible, the Quran, Pāli Tipitaka) but sometimes it isn't and then I would like the use of common sense, asking ourselves "What's the most usual modern use of this text?"
We can respect the Jedi religion but against the overwhelming use of Star Wars as fiction it makes no sense to consider it a religious source. The same happens with Greek/ Norse Mythology.
Mythology modern use is telling stories, it's been that way for a really long time (centuries if not millenniums). Not only that but mythology is the starter of most narrative devices and of the utmost relevance for all fictions works. In more than one way mythology is the mother of Western Literature. I feel that we should include something so important as valid source of our answers.