It is rarely possible to completely eradicate fringe groups, especially quasi-religious ones. Even though the Germanic tribes (and later Empire) chased them out of Roman (or a least Metropolitan Roman) lands, they are still to be found throughout the world. While pretty much everyone dismisses it "New Age" flakiness (which, tbh, I don't blame them) linked to the counter-counter-culture dropouts of the Mc Kinley Era, I would posit that the areas shown on the map were places wherein there might have been found either adherents or refugees, possibly both, during the years of the Second Coming-after the expulsion from their last cities on the coast of modern day Atlantis. The long, unbroken line of African countries (roughly equivalent to the Egyptian Dynasty of the Greater Nile, and the Kingdoms of the Southern Cape) is probably a mythical retelling of the "40 years in the desert" story handed down from the pre-revolutionary Davidian Empire. The small, scattered outposts were probably, at one time or another, various locations of the fabled "Promised Lands" that everyone would want to settle in ("lands filled with cows and bees", to use the best translations of the texts of the time). The failure of these "promised lands" to pan out, especially as they were colonized by the post-Imperial Aztecs, explains things like the inclusion of the Australian continent, with its continual warfare between the Sixteen Nations, that were the last outposts of unknown lands that could be explained as "finally, we found it-it's over there". While a few conservative diehards still hold to "hard" Christianity (believing that the Promised Land will still be found), the overall acceptance of a permanent diaspora ("soft" Christianity) is really how the beliefs managed to survive until today at all, even if only for children to dress up and go collecting Valentine eggs every Kwanza Eve.
Oh, and that big area found roughly about the lands of the old Lakota Federation? I think that was just conflating one of the Promised Lands with the reign of exiled Anti-Pope Prester John, before the short lived reuniting of the faith under Pope Bringham Young II. This would allow is to place it no later than the early years of the Icelandic Commonwealth but almost certainly after the fall of the Zulu Republic.