There was never a truly unified 'norse religion', and the pantheon was very flexible as well. Some elements, however, have roots into deep prehistory (a thunder god, layered worlds, you can name them), many of which have things in common throughout the Indo-European speaking world. These are the things that are shared between mythologies from Iceland, Greece, and India (except for the random coincidences of course). To what extent the Edda was composed of older elements and newer additions should be evaluated on a case-by-case basis. Odin is a good example; we have good indications that he did not exist before the Roman period, coming to prominence sometime around the 7th century or so. Similarly, a tribal god like Saxnot does not occur in the Edda.