Here is an interesting article that you should read (h/t Andrew Sullivan). In this article, Anthony Gottlieb is doing several things. He argues that we generally take the religion of our parents (or about 3/4s of us do), and that there is a strong link, beyond the obvious, between religion and fertility. More conservative religions maintain larger family sizes, and at the macro-level, these conservative religions are outstripping their liberal, secular, and atheistic counterparts in population growth around the world.
All of this to answer why a (male!?) Martian would care about the geographical loci of religions.