The other problem is, how do you make the map? Do you make a separate map for every religion? Religions aren't like realms, where each region is owned by one and only one. Any given region could potentially have followers from every single religion on a continent, in varying proportions. How do you represent that?
Make a color map. Each religion is represented by a pure (high excitation purity) color. It's very clear when you have three religions and you only have primary colors; it can be quite clear up to seven religions where you pick the rainbow colors. Of course, at 28 religions it's not clear at all, but you could argue outsider's don't make much of a difference anyway.
The regions where many religions are present are represented by a lower-saturation mix. Pagan regions are black. Regions where all religions are present are white.
But, this only mean that it could be done, not that it should be done....
