As Historians See It ---
From Philip Schaff's HISTORY OF THE CHRISTIAN CHURCH, Vol.2, p.19-f.
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"It is a remarkable fact that after the days of the Apostles no names of great missionaries are mentioned till the opening of the middle ages, when the conversion of nations were effected or introduced by a few individuals as St. Patrick in Ireland, St. Columba in Scotland, St. Augustine in England, St. Boniface in Germany, St. Ansgar in Scandinavia, St. Cyril and Methodius among the Slavonic races. There were no missionary societies, no missionary institutions, no organized efforts in the ante-Nicene age; and yet in less than 300 years from the death of St. John the whole population of the Roman empire which then represented the civilized world was nominally Christianized.
To understand this astonishing fact, we must remember that the foundation was laid strong and deep by the apostles themselves. The seed scattered by them from Jerusalem to Rome, and fertilized by their blood, sprung up as a bountiful harvest. The word of our Lord was again fulfilled on a larger scale: "One soweth, and another reapeth. I sent you to reap that whereon ye have not labored: others have labored, and ye are entered into their labor." (John 4:38)
Christianity once established was its own best missionary. It grew naturally from within. It attracted people by its very presence. It was a light shining in darkness and illuminating the darkness. And while there were no professional missionaries devoting their whole life to this specific work, every congregation was a missionary society, and every Christian believer a missionary, inflamed by the love of Christ to convert his fellow-men. The example had been set by Jerusalem and Antioch, and by those brethren who, after the martyrdom of Stephen, "were scattered abroad and went about preaching the Word." Justin Martyr was converted by a venerable old man whom he met walking on the shore of the sea. "Every Christian laborer," says Tertullian, "both finds out God and manifests him, though Plato affirms that it is not easy to discover the Creator, and difficult when He is found to make him known to all."
Celsus scoffingly remarks that fullers and workers in wool and leather, rustic and ignorant persons, were the most zealous propagators of Christianity, and brought it first to women and children. Women and slaves introduced it into the home-circle.
It is the glory of the gospel that it is preached to the poor and by the poor to make them rich. Origen informs us that the city churches sent their missionaries to the villages. The seed grew up while men slept, and brought forth fruit, first the blade then the ear, after that the full corn in the ear. Every Christian told his neighbor, the laborer to his fellow-laborer, the slave to his fellow-slave, the servant to his master and mistress, the story of his conversion as a mariner tells the story of the rescue from shipwreck."