To the first Christian leaders, it was for the Jews (only). And God had to scatter them to even get them to leave the vicinity of Jerusalem. Eventually, those who had lived in the Diaspora (like Paul) began the systematic extension of Christian teaching to the Jews scattered throughout the eastern Mediterranean. They stumbled over the Gentiles along the way, and it took a lot of hassle before the Gentiles were seen as a legitimate target for Christian missions.
Fletcher takes up the tale there. Christianity was an urban religion. An urban, Roman (as in, the Roman empire, not ethnic Romans) religion. It was fairly lower class, but became upper class rapidly after the legalization of Christianity under Constantine. Christianity became the religion of the Roman elites. Both Roman elites and Roman underclasses, however, saw themselves as Roman, and Romanitas was, in some ways, quite as central to their identity as their Christianitas.
In fact, the first major push to convert the heathen -- and it took some big personalities to push it, like Martin of Tours -- was the evangelization of the countryside. Pagan comes from a word meaning "country bumpkin," although in the late imperial age, the word "rustic" (rusticus) was more common. (The later word "heathen" comes from the Old English word for "heather," meaning someone who lived rough, rather than in towns.) A lot of people thought trying to teach rustics anything was a waste of time, but the elites (many of whom had country estates) were told it was their duty to bring the faith to their dependents, so the Church began (reluctantly) to convert the rubes.
Foreign missions -- that is, outside the empire -- were typically by request only. Roman merchants or war captives wound up forming a pocket of expatriates in some (so to speak) godforsaken place, and they would ask to be sent a bishop, so they could practice their faith in their new home. This is how Ulfilas got sent to the Goths and how Palladius got sent to the Irish. Of course, in their new home, the faith would slowly spread, just as it had originally spread from the Jews in the Diaspora to their Gentile neighbors.
Here's the astonishing thing. Fletcher says the first person we have record of who ever articulated a call to go convert some foreign pagans just because they were foreign as well as pagan, was Patrick. His call to evangelize the Irish was to go to the real Irish, the pagan Irish, those who had held him captive in his teens. He didn't intend to go just to minister to a community in exile; he was going to go make converts of those who hadn't asked him to. Apparently, nobody had had this thought before.
We lose this because we read back into history the missionary impulse we are familiar with. "We've a story to tell to the nations," we sang last Sunday. We assume that everybody from Pentecost on intended this to happen. But they didn't. It took over four hundred years before anybody thought that we ought to go barge in on people who hadn't asked us to in order to share our Christ with them.
I'm glad Patrick and his successors did so. But it makes me a little more tolerant of the people who don't "get it" in our churches today. It's natural to suppose that our primary concern centers around the way we do Church, and we conflate "American" or "Western" and "Christian" very easily. It's natural. You have to be taught to think otherwise. You have to be taught to imagine the possibilities of widening the circle that surrounds and defines us.