The following is a great quote from “Mission and Expansion of Christianity in the First Three Centuries” by Adolph von Harnack (1851-1930), from 1908 translated edition.  This quote is from Volume 3, chapter 1 (section 366-369).  The broader passage and related passages are available at the Christian Classics Ethereal Library (see http://www.ccel.org/ccel/harnack/mission.v.i.html#fna_v.i-p2.1)

Passage emphasizes the role of the common people in the early church as “missionaries”. We have tended to fall into the trap of celebrity. We focus on “heroes of the faith” and connect great works of God to “great people”. The church has been built on the foundation of Christ (and the twelve) but is built up from building blocks of very ordinary (yet extraordinary) followers of Christ.

“The most numerous and successful missionaries of the Christian religion were not the regular teachers but Christians themselves, in virtue of their loyalty and courage. How little we hear of the former and their results! How much we hear of the effects produced by the latter! Above all, every confessor and martyr was a missionary; he not merely confirmed the faith of those who were already won, but also enlisted new members by his testimony and his death. Over and again this result is noted in the Acts of the martyrs, though it would lead us too far afield to recapitulate such tales. While they lay in prison, while they stood before the judge, on the road to execution, and by means of the execution itself, they won people for the faith. Ay, and even after death. One contemporary document (cp. Euseb. vi. 5) describes how Potamiæna, an Alexandrian martyr during the reign of Septimius Severus, appeared immediately after death even to non-Christians in the city, and how they were converted by this vision. This is by no means incredible. The executions of the martyrs (legally carried out, of course) must have made an impression which startled and stirred wide circles of people, suggesting to their minds the question: Who is to blame, the condemned person or the judge?  Looking at the earnestness, the readiness for sacrifice, and the steadfastness of these Christians, people found it difficult to think that they were to blame. Thus it was by no means an empty phrase, when Tertullian and others like him asserted that the blood of Christians was a seed.

Nevertheless, it was not merely the confessors and martyrs who were missionaries. It was characteristic of this religion that everyone who seriously confessed the faith proved of service to its propaganda. Christians are to “let their light shine, that pagans may see their good works and glorify the Father in heaven.” If this dominated all their life, and if they lived according to the precepts of their religion, they could not be hidden at all; by their very mode of living they could not fail to preach their faith plainly and audibly. Then there was the conviction that the day of judgment was at hand, and that they were debtors to the heathen. Furthermore, so far from narrowing Christianity, the exclusiveness of the gospel was a powerful aid in promoting its mission, owing to the sharp dilemma which it involved.

We cannot hesitate to believe that the great mission of Christianity was in reality accomplished by means of informal missionaries. Justin says so quite explicitly. What won him over was the impression made by the moral life which he found among Christians in general. How this life stood apart from that of pagans even in the ordinary round of the day, how it had to be or ought to be a constant declaration of the gospel—all this is vividly portrayed by Tertullian in the passage where he adjures his wife not to marry a pagan husband after he is dead (ad Uxor., II. iv.-vi.). We may safely assume, too, that women did play a leading role in the spread of this religion (see below, Book IV. Chap. II.). But it is impossible to see in any one class of people inside the church the chief agents of the Christian propaganda.”