You can't tell a Christian from a non-Christian by where he lives or the way he speakes or how he dresses. There are no "Christian towns", there is no "Christian language", and they eat, drink and sleep just like everybody else. Christians aren't particularly clever or ingenious and they haven't mastered some complicated formula, like the followers of some religions.
But while it's true that they live in cities next to other people, and follow the same pattern of life as they do, in fact they have a unique citizenship of their own. They are, of course, citizens of their own lands - loyal ones, too. But yet they feel like visitors. Every foreign country is their homeland, and their homeland is like a foreign country to them ... They are nationals of various states, but citizens of heaven.
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 parts 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.
The Letter to Diognetius