My view is that the Church in its earliest days had a mission to reach all the Jews in the diaspora. Once that was largely accomplished (and the Church began to become populated by Gentiles), no real sense of "mission" (as we commonly term it) prevailed. Rather, the Church constituted a parallel, rival Order of society. There was The World, which Christians largely withdrew from, and there was The Church, which constituted a world-within-the-World, an outpost or embassy of the kingdom-to-come (I am tempted to use the diplomatic lingo of "mission" for the Church here). One was either in the World or in the Church, and becoming a Christian was a kind of individual or familial transfer of allegiance from one to the other.
Two things happened to disrupt this tidy parallelism, and they happened at more or less the same time. One was the conversion of Constantine in 312 and his attempt to direct the Church toward the task of helping to order society. This culminated in Theodosius making Christianity the State religion seventy years later. The other, which partially preceded Constantine and was greatly accelerated by the Church's response to Constantine, was the evangelization of the countryside. As Richard Fletcher points out in The Barbarian Conversion: from paganism to Christianity, the first "outside" group identified as a target population to which missionaries were to be sent and for which churches were to be built were the rural inhabitants of the Roman Empire.
Christianity was largely a town affair back in those days, and the superstitious "rustics" had hitherto been thought unreachable. But now, wealthy Christian landowners who employed the country folk were urged to build churches and fund evangelistic work. Bishops urged each other to get on with sending priests to teach the faith, to baptize, and to challenge local pagan practices.
Over the centuries, successive populations were targeted for missionary work. First, the unchristian inhabitants of one's own country, then the unchristian inhabitants of other countries. This meant, in one sense, bringing them into the Church; but it also meant bringing them into the new understanding of society, into "Christendom." Romanizing the barbarians and converting the heathens were often considered synonymous terms.
My own prejudice finds the best exemplars of early missions work to be done in what are often called the Dark Ages. Irish monks spreading the faith by way of missions to the rest of Europe had more of Christ to share and less of (our) World. English churchmen sending priests to other Anglo-Saxon kingdoms, then Frisia and other Germanic tribes may have been dismissive of paganism, but they saw the tribes and kingdoms they were preaching to as culturally kin, so (at least until Charlemagne came along) it was a more equal sharing than it sometimes later became.
Still, even if you purge the Church of the whole idea of trying to monopolize society (where we are seen as sharing, not Christ but American Christendom), it still remains that the Church constitutes a rival way of ordering existence. The mission is to invite others into that way and make them at home there. And, quite apart from the challenge of not merely importing our culture to other lands when we share Jesus, the challenge remains to "keep ourselves unspotted from the world," ourselves.