Nothing is known of its author, and little of Diognetus, the enquirer it answers. Most scholars date it to the second century, and it is usually printed among the Apostolic Fathers, with the Didache and the letters of St Ignatius and St Clement.
The writer answers Diognetus' questions one by one. The idols of the Greeks are wood and stone; the sacrifices of the Jews, offered to a God who needs nothing, miss the point. Then he describes the Christians themselves, in the passage that has made the letter famous.
Christians, he writes, do not live in cities of their own or speak a strange language. They live in their own countries, but as sojourners. They marry and have children, but do not cast their children away. They obey the laws, and surpass them by their lives. They are in the world as the soul is in the body: spread through every part of it, not of it, and holding it together.
The letter then turns to why God sent His Son, not to rule by force but in gentleness, and why He waited so long: to let us see that we could not save ourselves. Its only manuscript, copied in the thirteenth or fourteenth century, was destroyed by fire in Strasbourg in 1870, but copies made before then survive.