Detail

Here it is, a detailed view of the big three world religion/culture regions.

Christianity was split twice, first by the Great Schism of 1054, and later by the Protestant Reformation (1517-1648). Orthodox Christians, with the Russian army, moved east to conquer Siberia. By contrast, the Catholic powers moved by sea, and attempted to convert the world to Catholicism while trying to stop the Muslim Ottomans and crush the Protestant Reformation. Latin America and the Philippines are Catholic due to the zeal of the missionaries sent there. The Protestants got a big break when Plymouth colony was established in Massachusetts. Then, under the British empire, they came to Australia and recolonized Canada from its original French Catholic background.

After the Napoleonic Wars came the scramble for Africa. Missionaries here however were stopped by the harsh jungle terrain, hostile natives and tropical disease. Though these things also exist in South America, missionaries there have had far more time to preach and be successful. As a result, Africa was never fully converted to Christianity (though today many missionaries are trying to change that).

Though America began as a Protestant country, Catholic immigrants began to arrive. The first were the Irish, followed by Italians and Germans. Whereas the Germans moved quickly into places like Minnesota, the Italians and Irish remained on the coast, in New York and Boston. Today, Catholic Mexicans continue to enter into the US, settling just north of the border.

Islam is a religion that also split (around 670), though not in the same way as Christianity. Today, the Shia mostly live in the country of Iran. Most other countries are dominated by Sunni Muslims.
The far northern extent of Islam corresponds to the Battle of the River Talas (751) where Chinese armies attempting to advance west on the silk road were defeated by the armies of the Caliph. Interestingly, there are no areas encompassed by Islam that haven't been converted yet, even though their territory comprises the hottest regions on Earth.

The lands of the east are mostly Buddhist, although the India and Nepal are primarily populated by people following the Hindu faith. Hinduism is the root from which Buddhism springs, just as Judaism is the root from which Christianity and Islam spring. Southeast Asia is of the Theravada Buddhism, whereas to the north, China is of the Mahayana Buddhism. Tibet and Mongolia are of the Vajrayana Budddhism, a subset of Mahayana. Interestingly, Mahayana Buddhism was formed when Buddhist missionaries arrived in Afghanistan, then under the rule of Greeks due to Alexander's conquest of the persian empire. Here, some claim that Greek ideas were added to those of the Buddha. This religion then traveled north, along the silk road to China where it remains today.

The followers of Judaism have had a long and drawn out history. After their revolt against the Roman Empire, they were expelled from the Holy Land. Their people were scattered over the length and breadth of the earth, sometimes serving as rulers in locations as far flung as Kazanstan, Yemen, Crimea, Tunis and Morocco. In the Christian world, they suffered the abuse known as the pogrom. Their darkest hour was at the hands of Nazi death squads, less than a century ago. Today, about half of the people live fulfilling lives in Europe and the United States. The other half lives in Israel.

The dark blotches are areas where neither of the three religions are practiced. These are frontier areas, cut off by geography and are often nearly impossible to get to or to supply. These areas have remained and will continue to remain backwaters of civilization, underdeveloped, pristine and lacking the urge to develop.

Edit: newer version at [link]

2/19/12 EDIT:
map base source [link]
map base created by Lokal_Profil [link]