From Christianity Today:
Although its authorship remains in doubt, "The Epistle to Diognetus" is undoubtedly one of the finest works of early Christian apologetics. It was written in the second or third century to Diognetus, who was either a prominent Roman or a creature of the author's imagination. In any case, the letter's practical intent was to serve as an apologetic to the educated classes of the Roman Empire. The letter, written when the Roman Empire was pagan, speaks to our day in two ways. First, it suggests that the Christian faith can prosper even in a hostile or indifferent regime—that is, even after Christendom. Second, it reminds Christians—especially those who lament their status as victims or outsiders—that the world is not our home and we should be prepared to endure persecution gracefully.
The following excerpt is a wonderful reminder of how we should live as Christians in the world today. Notice the lack of racism, the sanctity of life, the sacredness of sexuality and the generous spirit of these early believers. Such grace in spite of intense persecution provides us a model for these post-christian days. If we are to impact the world as they did, we would do well to imitate their life and faith.
For the Christians are distinguished from other men neither by country, nor language, nor the customs which they observe. For they neither inhabit cities of their own, nor employ a peculiar form of speech, nor lead a life which is marked out by any singularity. The course of conduct which they follow has not been devised by any speculation or deliberation of inquisitive men; nor do they, like some, proclaim themselves the advocates of any merely human doctrines. But, inhabiting Greek as well as barbarian cities, according as the lot of each of them has determined, and following the customs of the natives in respect to clothing, food, and the rest of their ordinary conduct, they display to us their wonderful and confessedly striking method of life. They dwell in their own countries, but simply as sojourners. As citizens, they share in all things with others, and yet endure all things as if foreigners. Every foreign land is to them as their native country, and every land of their birth as a land of strangers. They marry, as do all [others]; they beget children; but they do not destroy their offspring. They have a common table, but not a common bed. They are in the flesh, but they do not live after the flesh. They pass their days on earth, but they are citizens of heaven. They obey the prescribed laws, and at the same time surpass the laws by their lives. They love all men, and are persecuted by all. They are unknown and condemned; they are put to death, and restored to life. They are poor, yet make many rich; they are in lack of all things, and yet abound in all; they are dishonoured, and yet in their very dishonour are glorified. They are evil spoken of, and yet are justified; they are reviled, and bless; they are insulted, and repay the insult with honour; they do good, yet are punished as evil-doers. When punished, they rejoice as if quickened into life; they are assailed by the Jews as foreigners, and are persecuted by the Greeks; yet those who hate them are unable to assign any reason for their hatred.To sum up all in one word--what the soul is in the body, that are Christians in the world.