"For 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 way of life…But 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 citizenship. They live in their own countries, but only as nonresidents; they participate in everything as citizens, and endure everything as foreigners. Every foreign country is their fatherland, and every fatherland is foreign. They marry like everyone else, and have children, but they do not expose their offspring. They share their food but not their wives. They are in the flesh, but they do not live according to the flesh. They live on earth, but their citizenship is in heaven. [Emphasis mine] They obey the established laws; indeed in their private lives they transcend the laws. They love everyone, and by everyone they are persecuted. They are unknown, yet they are condemned; they are put to death, yet they are brought to life. They are poor, yet they make many rich; they are in need of everything, yet they abound in everything. They are dishonoured, yet they are glorified in their dishonour; they are slandered, yet they are vindicated. They are cursed, yet they bless; they are insulted, yet they offer respect. When they do good, they are punished as evildoers; when they are punished, they rejoice as though brought to life. By the Jews they are assaulted as foreigners, and by the Greeks they are persecuted, yet 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 out 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."
Letter to Diognetus 5:1-6:4
in Holmes, Michael W. (ed. trans), The Apostolic Fathers: Greek Texts and
English Translation (3rd ed.), Grand Rapids, Michigan: Baker Academic, 2007, pp.701-5