The purpose of this article is to survey what the Epistle to Diognetus says about Christian ethics and the Christian life. It is distinct from paganism and Judaism. What the author does not admit is that a pure ethic, such as in found in the prophets of the Old Testament, is quite compatible with a Christian ethic. What distinguishes the Christian way of life is not something external, but something spiritual. 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.
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 live on earth, but their citizenship is in heaven. They love everyone, and by everyone they are persecuted. The Epistle to Diognetus then compares the relationship of the church to the world with that of the soul to the body.