Schaff, Philip. History of the Christian Church (The Complete Eight Volumes in One). Amazon Kindle Edition, 2014.
Volume 2, Ante-Nicene Christianity A.D. 100-325, “Chapter 13. Ecclesiastical Literature of the Ante-Nicene Age, and Biographical Sketches of the Church Fathers.” sec. 159-204.
§ 170. The Epistle to Diognetus.
After a fairly comprehensive bibliography, Schaff notes that this letter “was unknown in Christian literature until Henry Stephens, the learned publisher of Paris, issued it in Greek and Latin in 1592, under the name of Justin Martyr” (Schaff 2014, loc. 21324). Though there is considerable doubt about the provenance of the text, Schaff considers it to be a genuine second century document, brilliant in its composition.
The letter, addressed to Diognetius, describes Christianity, the reason why Christians can look on the world with contempt, face death with courage, and love one another (Schaff 2014, loc. 21331). Schaff suggests the recipient may have been the Stoic tutor of Marcus Aurelius, who influenced him around 133 AD.
The letter is twelve chapters long, beginning with an address and compliments to Diognetus, a Gentile. It speaks of the nature of idols, of Jewish superstitions, then of the nature and strength of the Christian life (Schaff 2014, loc. 21339). The author speaks of the condition of the word without Christ, the blessings of his coming, and the hope found in Christ (Schaff 2014, loc. 21347).
Schaff considers the authorship, concluding that, thoguh the manuscript ascribes the letter to Justin Martyr, the style is different. Authorship is not actually known, but Schaff again places the letter in the second century, probably about the time of Marcus Aurelius (Schaff 2014, loc. 21360).