Christians are not differentiated from other people by country, language, or customs; you see, they do not live in cities of their own, or speak some strange dialect, or have some peculiar lifestyle.
They live in both Greek and foreign cities, wherever chance has put them. They follow local customs in clothing, food and the other aspects of life. But at the same time, they demonstrate to us the wonderful and certainly unusual form of their own citizenship.
They live in their own native lands, but as aliens; as citizens, they share all things with others; but like aliens suffer all things. Every foreign country is to them as their native country, and every native land as a foreign country.
They are treated outrageously and behave respectfully to others. When they do good, they are punished as evildoers; when punished, they rejoice as if being given new life. They are attacked by Jews as aliens, and are persecuted by Greeks; yet those who hate them cannot give any reason for their hostility.
To put it simply - the soul is to the body as Christians are to the world. The soul is spread through all parts of the body and Christians through all cities of the world. The soul is in the body but is not of the body; Christians are in the world but not of the world.
*Excerpts from the 2nd Century "Letter to Diognetus".
Quoted in Darrell L. Guder, et al., Missional Church: A Vision for the Sending Church in North America (Grand Rapids, Eerdmans, 1998), 120.