For most of known “history”, the American continents were unknown. For most of history, Italy was “the west”, Spain was “the far west”, and the British Isles were semi-legendary island nations off the map, in the vast Atlantic Ocean. The “center” of the world was either in Rome (if you were Roman), Constantinople (if you were Greek, China, if you were Chinese, or Baghdad or thereabouts, if you were Persian. This got adjusted a bit with the onset of Islam, which listed Mecca, in modern-day Saudi Arabia as being the center of the earth. For many Christians, the Center has always been Jerusalem.
Even today, it is customary in much of Africa and Asia to put the letters “J.P.” after your name if you’ve made a pilgrimage to the Holy Land. You see it in ordinary correspondence in much the same was as we see “MD” or “Ph.D.” or “C.P.A” in the West. This writer has a copy of a medieval map showing the world as a trefoil. At the center is Jerusalem; to the east there is a “petal” labelled “Persia”, to the south, one labelled “Africa”, and to the west, one labelled “Rome”. Just a little thought will show that this was a common way to think of Christendom in the five hundred years following Constantine and the Council of Nicaea. Jerusalem was at the center and Rome, Antioch and Alexandria were the cities that mattered surrounding it.
So, it should come as no surprise that the missionary activities of the Church fell into a very formatted pattern, especially after 650 or so, when Islam arose to cut off expansion to the southeast. Nestorian Christians built a further wall to the east. While the Nestorian Church was still Christian, and considered the Bishop of Antioch to be its head, they in fact paid no attention to him. The Nestorian Church expanded vastly across the mainland of Asia for five hundred years or more, but because the Ecumenical Patriarch in Constantinople called them heretics, they were really an independent Christian Church.
Alexandria pushed south. Again, Islam, in the east, shut off expansion in that direction (though many would (and already have) argued that Islam itself is simply a Christian heresy: Monarchianism), and so the Coptic Church pushed south along the Nile into sub-Saharan Africa, Ethiopia, and the “horn” of Africa. With Rome effectively stopping much expansion to the west, the Greek Church pushed north, converting many of the Slavic tribes in what we, today, call Russia, the Balkans, the Ukraine, the Caucasus, Georgia and so forth. Rome initially occupied North Africa, but couldn’t go further south because of the forbidding Sahara Desert. Eventually, the Bishop of Rome faced up to the fact that to build the size of his Church, he was going to have to subvert the other three that already existed in his area of responsibility: France/Spain (the Gallican Church), the remaining Arians among the Germans, and Britain. To this effort, the Roman Popes devoted the next five hundred years. It was made somewhat easier because the Arians, an outright Christian heresy that had taken root in much of Germany and Central Europe, had been condemned by the first Council. That being the case, the Popes could attack the Gallican and British missionaries, pretending that they were also heretics.