Maybe I'm just not looking in the right places, but it seems hard to find really detailed overviews of the religions of the world. This world map from Wikipedia --
-- is above average, but still leaves something to be desired. Here's a link to the Web location of the map, in case you want a slightly larger view.
So. The religions shown on the map are Christianity, Islam, Hinduism, Buddhism, Judaism, Chinese religion, Korean religions, Shinto, Folk religions and No religion.
"Folk religions" is a somewhat condescending term. It amounts to "etc." That's one problem. A few decades ago in many Western surveys this problem would have been worse, with "Chinese religion," "Korean religions" and Shinto more likely to have been included in the miscellaneous "folk religions."
Another big problem with this map is that there are no "folk religions" shown in all of Africa and all of South America. And that's inaccurate.
The map is also missing some religions somewhat more familiar to the West, and therefore typically somewhat less condescendingly-discussed by Westerners, than the world's various "folk religions" : Sikhism, Bahá'í, Jainism and Zoroastrianism, to name a few of which I am not ignorant. Zoroastrianism has shrunk to 200,000 adherents worldwide or less, making it somewhat hard to show it on a world map. Its absence is somewhat more striking on those videos of world maps where patches of color grow and shrink to show the growth and decline of religions: Zoroastrianism was the state religion of Iran for over 1000 years before being surpassed by Islam.
And what are those videos supposed to do about ancient Rome? Describing it as "pagan" is, I suppose, more accurate than simply leaving the area blank and thereby implying that there was no religion there at all until the spread of Christianity. But "paganism" is often used, like "folk religions," as a sort of condescending "etc" by people who don't know what was going on and don't care. What was going in pre-Christian Rome was a great variety of religions existing side by side. The official Roman position was to respect all religions. A Roman could belong to as many religions as he or she liked. The Romans expected in return that all religions would respect each other, and this was a sort of built-in conflict with montheistic religions.
But it's sort of hard to show a region with hundreds of different religions on a world map. (And the number of religions practiced in the city of Rome alone may have cracked 1000.) My purpose with this post is not to criticize the makers of maps of world religions but to show how difficult, and ultimately impossible, their task is. Especially in times and places such as ancient Rome, and increasingly again in our time, in which the emphasis is less on imposing one religion and more on tolerating all religions, making for cultural phenomena whose complexity maps are not suited to show.