For Christians are not distinguished from the rest of mankind either in locality or in speech or in customs. For they dwell not somewhere in cities of their own, neither do they use some different language, nor practise an extraordinary kind of life. Nor again do they possess any invention discovered by any intelligence or study of ingenious men, nor are they masters of any human dogma as some are. But while they dwell in cities of Greeks and barbarians as the lot of each is cast, and follow the native customs in dress and food and the other arrangements of life, yet the constitution of their own citizenship, which they set forth, is marvellous, and confessedly contradicts expectation. They dwell in their own countries, but only as sojourners; they bear their share in all things as citizens, and they endure all hardships as strangers. Every foreign country is a fatherland to them, and every fatherland is foreign. They marry like all other men and they beget children; but they do not cast away their offspring. They have their meals in common, but not their wives. They find themselves in the flesh, and yet they live not after the flesh. Their existence is on earth, but their citizenship is in heaven. They obey the established laws, and they surpass the laws in their own lives. They love all men, and they are persecuted by all. They are ignored, and yet they are condemned. They are put to death, and yet they are endued with life. They are in beggary, and yet they make many rich. They are in want of all things, and yet they abound in all things. They are dishonoured, and yet they are glorified in their dishonour. They are evil spoken of, and yet they are vindicated. They are reviled, and they bless; they are insulted, and they respect. Doing good they are punished as evil-doers; being punished they rejoice, as if they were thereby quickened by life. War is waged against them as aliens by the Jews, and persecution is carried on against them by the Greeks, and yet those that hate them cannot tell the reason of their hostility.
For the rest, see: http://deovivendiperchristum.wordpress.com/2013/09/16/epistle-to-diognetus-2nd-century-on-the-christian-life-in-the-world-but-not-of-it/