WE owe the text of this work to a single MS of the thirteenth or possibly the fourteenth century, now no longer extant. This MS had originally belonged to Joann. Reuchlin († 1522), and ultimately found a home in the Strassburg Library, where it perished by fire during the Franco-German war in 1870 together with the other manuscript treasures contained therein. Two transcripts however had been made at the close of the sixteenth century, one by H. Stephens (in 1586), who first edited the Epistle to Diognetus (Paris, 1592), and another by Beurer (1587—1591), who however did not publish it. Stephens' copy is now at Leyden; that of Beurer is lost, but some of its readings are preserved by Stephens and by Sylburg (1593). Happily the portion of the Strassburg MS containing this Epistle was carefully collated by E. Cunitz in 1842 for Otto's first edition of Justin Martyr (1843), and again by E. Reuss still more accurately in 1861 for the same editor's third edition (1879).
The Strassburg MS contained several spurious or doubtful writings of Justin Martyr, at the close of which was the Epistle to Diognetus, likewise ascribed to him, τοῦ αὐτοῦ [ Ἰουστίνου φιλοσόφου καὶ μάρτυρος ] πρὸς Διόγνητον, besides other works following—some of them in a later hand— with which we are not concerned. Hence subsequent writers ascribed it unhesitatingly to Justin. Tillemont was the first (1691) who threw any doubt on this ascription. More recently critics, one and all, have agreed to assign it to some other author. It is not mentioned by Eusebius, or in any other ancient account of Justin's works; and its style is wholly different from that of Justin.
The most diverse opinions have been held respecting its date. Almost every epoch from the middle of the second century to the reign of Constantine in the beginning of the fourth has been assigned to it; nor indeed is any certainty possible. On the whole, however, the earlier date (c. A.D. 150) seems the more probable. Its ascription to Justin Martyr and its companionship with early writings in the ms suggest an epoch not later than the first half of the second century. The person meant by Diognetus is not improbably the tutor of Marcus Aurelius, here addressed as an enquirer after truth. The reference to the emperor commissioning his son (c. 7 ὡς βασιλεὺς πέμπων υἱὸν βασιλέα), as illustrating the great truth of Christian theology, may not improbably have been suggested by such events as the adoption of M. Aurelius by Antoninus Pius into the tribunician power (A.D. 147), or the association of his adopted son L. Aelius (A.D. 161) or of his own son Commodus (A.D. 176, 177) in the empire by M. Aurelius himself. The simplicity in the mode of stating theological truths, and the absence of all reference to the manifold heresies of later times, both point to a somewhat early date. Whenever it was written, it is one of the noblest and most impressive of early Christian apologies in style and treatment.
The dream of some very recent writers who suppose it to have been written, or rather forged, at the revival of learning in the sixteenth century may be dismissed at once as inconsistent alike with its style and contents, and with the history of the documents as given above.
The Epistle to Diognetus, however, does not reach beyond the tenth chapter, where it ends abruptly. The two remaining chapters belong to some different work, which has been accidentally attached to it, just as in most of the extant mss the latter part of the Epistle of Polycarp is attached to the former part of the Epistle of Barnabas (see above, pp. 166 sq, 242), so as to form in appearance one work. Probably in this case also an archetypal ms had lost some leaves. Of this there seems to have been some indication in the Strassburg MS itself.
Who then was the author of this latter work? May we not hazard a conjecture which may be taken for what it is worth? The writer was Pantænus, the master of Clement (c. A.D. 180—210). Clearly it is Alexandrian, as its phraseology and its sentiments alike show. More especially he treats the account of the creation and the garden of Eden (c. 12 παράδεισος τρυφῆς κ.τ.λ.) spiritually of the Church of Christ; and Pantænus is singled out with two or three other early fathers by Anastasius of Sinai in two passages as exhibiting this mode of treatment (ed. Migne, p. 860, p. 962). Nor indeed could any one more appropriately use the words (c. 11) ἀποστόλων γενόμενος μαθητὺς γίνομαι διδάσκαλος ἐθνῶν of himself than Pantænus the Apostle of the Indies. The first part of the sentence, ἀποστόλων μαθητής, wrongly understood, has given a place to the Epistle to Diognetus as a whole among the Apostolical Fathers, though (as we have shown) the last two chapters form no part of that Epistle. It is perhaps this very sentence also, or similar language of Pantænus elsewhere, which has led to the impossible statement in Photius (Bibl. 118) that Pantænus himself had listened to the preaching of the apostles.