The Epistle to Diognetus is an apology for Christianity,  written by an unknown writer to a pagan of high social or political rank.   It probably dates from the 2^nd or 3^rd century.  In chapters 5 & 6 Christians are described as the soul of the world.

“Christians are not distinguished from the rest of mankind by either country, speech, or customs; the fact is, they nowhere settle in cities of their own; they use no peculiar language; they cultivate no eccentric mode of life.  certainly, this creed of theirs is no discovery due to some fancy or speculation of inquisitive men; nor do they, as some do, champion a doctrine of human origin.  Yet while they dwell in both Greek and non-Greek cities, as each one’s lot was cast, and conform to the customs of the country in dress, food, and mode of life in general, the whole tenor of their way of living stamps it as worthy of admiration and admittedly extraordinary.  They reside in their respective countries, but only as aliens.  They take part in everything as citizens and put up with everything as foreigners.  Every foreign land is their home and every home a foreign land.  The marry like all others and beget children; but they do not expose their offspring.  Their board they spread for all, but not their bed.  They find themselves in the flesh, but do not live according to the flesh. They spend their days on earth, but hold citizenship in heaven. they obey the established laws, but in their private lives they rise about the laws. 

They love all men, but are persecuted by all. They are unknown, yet are condemned; they are put to death, but it is live that they receive.  They are poor, and enrich many; destitute of everything, they abound in everything.  They are dishonored, and in their dishonor find their glory.  The are calumniated, and are vindicated.  They are reviled, and they bless; they are insulted and render honor.  Doing good, they are penalized as evildoers; when penalized, they rejoice because they are quickened into life.  they Jews make war on them as foreigners; the Greeks persecute them; and those who hate them are at a loss to explain their hatred.

Vasily Perov’s painting illustrates clandestine meetings of Christians in pagan Kiev. (Photo credit: Wikipedia)

In a word: what the soul is in the body, that the Christians are in the world.  The soul is spread through all the members of the body, and the Christians throughout the cities of the world.  The soul dwells in the body, but is not part and parcel of the body; so Christians dwell in the world, but are not part and parcel of the world.  Itself  invisible, the soul is kept shut up in the visible body; so Christians are known as such in the world, but their religion remains invisible.  The flesh, though suffering no wrong from the soul, yet hates and makes war on it, because it is hindered from indulging its passions; so, too, the world, though suffering no wrong from Christians, hates them because they oppose its members; so, too, Christians love those that hate them.  The soul is locked up in the body, yet is the very thing that holds the body together; so, too, Christians are shut up in the world as in a prison, yet it is precisely they that hold the world together. 

Immortal, the soul is lodged in mortal tenement; so, too, Christians, though residing as strangers among corruptible things, look forward to the incorruptibility that awaits them in heaven.  The soul, when stinting itself in food and drink, fares the better for it; so, too, Christians, when penalized, show a daily increase in numbers on that account.  Such is the important post to which God has assigned, and they are not at liberty to desert it.”