An interesting Flash map can be found here that shows the historic spread of some of the worlds main religions.

It’s pretty interesting, but it could have been done better.

There’s no mention of any of the gods of ancient Greece and Rome, it also doesn’t cover any of the religions in the Americas prior to the European conquests (Or after; Mormonism, etc.) , nor does it mention Taoism, Shintoism, or Confucianism in east Asia.  Also, the map can be misleading when  you see huge swaths of North Africa become Islamic, and 2/3 of India becoming Hindu, as the population represented by the small Hindu area outnumbers the almost vacant North African desert regions.

Still though, not a bad first attempt.

If anything, iti llustrates religion, like eveything else, is a living organism controlled by Darwinistic fate.