The Epistle to Diognetus was written by an unnamed Christian to a high-ranking pagan, probably in the late second century. Perhaps the earliest preserved explanation and defense of the Christian faith to a nonbeliever, it provides insight as to how early Christians viewed the world and their place in it:
Christians are not distinguished from the rest of humanity by country, language, or custom. For nowhere do they live in cities of their own, nor do they speak some unusual dialect, nor do they practice an eccentric lifestyle. While they live in both Greek and barbarian cities, as each one’s lot was cast, and follow the local customs in dress and food and other aspects of life, at the same time they demonstrate the remarkable and admittedly unusual character of their own spiritual citizenship.
They live in their own countries, but only as aliens; they participate in everything as citizens, and endure everything as foreigners. Every foreign country is their fatherland, and every fatherland is foreign. They are “in the flesh,” but do not live “according to the flesh.” They live on earth, but their citizenship is in heaven. They obey the established laws; indeed, in their private lives they transcend the laws.
They love everyone. They are put to death, yet they are brought to life. They are in need of everything, yet they abound in everything. They are dishonored, yet they are glorified in their dishonor. They are slandered, yet they are vindicated. They are cursed, yet they bless. They are insulted, yet they offer respect. When they are punished, they rejoice as though brought to life. Those who hate them are unable to give a reason for their hostility.
In a word, what the soul is to the body, Christians are to the world. The soul is dispersed through all the members of the body, and Christians throughout the cities of the world. The soul dwells in the body, but is not of the body; likewise, Christians dwell in the world, but are not of the world. The soul, which is invisible, is confined in the body, which is visible; in the same way, Christians are recognized as being in the world, and yet their religion remains invisible.
— Epistle to Diognetus 5:1–6:3