What causes a religion to go from a "religion" to "mythology"? For example, what were the driving factors that pushed Norse mythology out once Christianity was rising? What are the chances of Christianity becoming mythology in the distant future?

I do not wish to discourage new answers, but I can present some links to earlier ones. This question, or at least parts of it, were answered here quite recently by u/itsallfolklore in this thread. The topic was also discussed some time ago, with a slightly different conclusion, by u/Spencer_A_McDaniel in this post on her blog.