These two levels are named the Lesser Mysteries and the Greater Mysteries and their delineation is quite obvious in Catholicism, Islam, and mystic Christianity. A story becomes a myth when it is crafted to reflect a universal truth. The listeners also say when a story becomes a myth, i.e. it has to communicate those universal truths. Time says so when a myth keeps caputuring the imagination for centuries or millennia, dressed up slightly differently for each culture or era. Myths are the stories we tell ourselves to explain the world around us and within us.
Myths are universal truths applicable regardless of era, culture, gender, age, etc.
Religion comes from the Latin word religare which means to bind . So a religion binds for the times those universal truths held in Myths and presents them to the culture of that particular religion. This is why behind the trappings of most religions you still find those basic truths of how the world works and how best to live in it with each other. Not everybody thinks the Bible is just a myth. Many see it as a compilation of history, legend, myth, folklore, poetry, and prophecy containing both universal elements and culture-specific elements.