There are two layers of misunderstand that can intrude when attempting to understand belief systems - contemporary and historical. What existed in pre-conversion Northern Europe was a largely unorganized belief system rather than a dogmatic, structured religion with a hierarchy of professional priests, dogma, and importantly, a written text. Referring to pre-conversion belief systems as religions conveys an idea to modern people that is misplaced.

Then there is a problem - again largely in perception - with the term "myth." Following is an answer (with a follow-up) that I posted over at /r/Folklore.

The core problem here is that people use the terms "myth" and "mythology" in very different ways. Your perception of when folklore overlaps with myth can occur just about anywhere depending on your definition of the terms - especially with the latter, which tends to lack the most focus.

Mythology is technically the study of myth, so what we really need to do is to understand what "myth" is. People tend to use the term in two ways: the first (and most appropriate) is to discuss ancient stories that were written down; the second is to describe third-world (or simply "other people's") stories and belief systems. This is problematic.

People talk about Indian myth or Polynesian myth even when there may be living people who tell the stories and adhere to that belief system. This strikes me as simply wrong. Consider the difference separating the "Resurrection story" and the "Resurrection myth." In this way, "story" simply describes something from the Bible; the "myth" can only be taken as an insult to Christians. It implies that there is a story about the resurrection of Jesus, but it would be silly to believe it. When we talk about Polynesian myth, we are demeaning a living belief system and its codex of oral stories.

For this reason, I do not use the term "myth" to describe living belief systems and their associated stories. That said, people still do use the word "myth" to describe modern traditions. To understand the term better, let's turn to the ancient world to see what was going on with the myths of various cultures.

In the ancient world, for various reasons people of various sorts (priests but also authors/poets) began writing down the stories that were being told at the time. "Stories that were being told at the time" can be understood to mean, the folklore - the oral traditions - of the time. These stories included heroic legends (Perseus, for example), historical legends (Troy, for example), etiological legends (about the creation of the world and the world order), and stories about encounters with powerful supernatural beings.

"Powerful supernatural beings" presents another problem for us because we tend to identify these entities with the same term that we use for the deity of monotheistic religions - that is, "God." Were these "powerful supernatural beings" exactly like the monotheistic "God"? Not really, but some were close while others were not.

In addition, these stories that were being told as expressions of the belief systems at the time. They were legends in the sense of "stories generally told to be believed." The written myths were a step removed from the stories because they were recorded and static, but they were a direct reflection of the folklore of the time.

Ancient myths, then, are a reflection of contemporary ancient folklore, just as the Grimm collection, recorded without electronic devices and often abridged and merging numerous variants are imperfect reflections of nineteenth-century German folklore. Either way, ancient myths are one of our best paths to understand the folklore of ancient cultures.

In a modern setting, myth is (improperly) used to describe the folklore of some people. We tend not to use the term to describe the stories in the Bible, but people do tend to use the term to describe "other people's religions."

To my eye, religious texts, regardless of the religion, can be seen as codifying a great deal of folklore. It is a step removed from folklore because of the way it has been fossilized or institutionalized, but folklore is often at its heart, and as such, the word "myth" either applies to it all, or it should not be used for anything that is modern.

edit: I am also reminded of something I used to ask my students: "What is the difference separating a cult from a religion?" The answer I gave after considerable discussion on the part of my students was "one or two centuries."

Mythology is not just another word for religion. Ancient Greek mythology isn't the same thing as ancient Greek religion, and the same goes for the Norse and the Egyptians. Mythology is just a part of a religion, specifically its body of myth - the legends, stories and folklore that informs the religion's beliefs and world view.

But there's more to practising a religion than just knowing all the stories. For example, when we're talking Greek mythology, we're probably talking about their legends of gods, heroes and monsters. But those myths do not alone comprise ancient Greek religion, which also includes rituals, sacrifices, festivals, and so on. Likewise, there absolutely is a Christian mythology, comprised largely of the stories in the bible, but there's more to Christian religion than simply reciting the literature.

Note that I am not using 'myth' to mean 'a story that is untrue' as in 'that's just a myth!' and I do not condone using the term to denigrate others' religions even though I am not religious myself.