Robert M. Grant describes the epistle of Mathetes to Diognetus (The Anchor Bible Dictionary, v. 2, p. 201): A late 2nd century apology addressed to a certain Diognetus who is otherwise unknown. It describes the Christians of the 2nd century.
Here are some direct quotes:
CHAPTER V — THE MANNERS OF THE CHRISTIANS.
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.
CHAPTER VI — THE RELATION OF CHRISTIANS TO THE WORLD.
To sum up all in one word–what the soul is in the body, that are Christians in the world. The soul is dispersed through all the members of the body, and Christians are scattered through all the cities of the world. The soul dwells in the body, yet is not of the body; and Christians dwell in the world, yet are not of the world. The invisible soul is guarded by the visible body, and Christians are known indeed to be in the world, but their godliness remains invisible. The flesh hates the soul, and wars against it, though itself suffering no injury, because it is prevented from enjoying pleasures; the world also hates the Christians, though in nowise injured, because they abjure pleasures. The soul loves the flesh that hates it, and [loves also] the members; Christians likewise love those that hate them. The soul is imprisoned in the body, yet preserves that very body; and Christians are confined in the world as in a prison, and yet they are the preservers of the world. The immortal soul dwells in a mortal tabernacle; and Christians dwell as sojourners in corruptible [bodies], looking for an incorruptible dwelling in the heavens. The soul, when but ill-provided with food and drink, becomes better; in like manner, the Christians, though subjected day by day to punishment, increase the more in number. God has assigned them this illustrious position, which it were unlawful for them to forsake.