In high school I remember doing a small unit in my English class on Norse mythology and I'm just wondering how it went from being a religion to mythology. I hope this isn't a dumb question, I know extrodinaraly little on the subject. Thanks!!
Technically speaking, every religion have mythology (a set of myths and stories), but in common use, the word mythology is typically used for the myths of religions which have no more active followers. So in short, Norse religion became mythology when the Norse became Christianized.
A living religion consists of much more that the myths and stories - it may have traditions, rituals, festivals, institutions, perhaps some kind of priesthood. But when a religion dies out, the rituals and institutions cease to exist. What remains is the mythology, at least if it was written down and preserved.