The epistle to Diognetus.
Counted among the apostolic fathers or Christian apologists.
The anonymous epistle to Diognetus, though obviously an ancient document, is not attested in any of the church fathers. The author is sometimes called Mathetes because he identifies himself as a disciple (μαθητης, transliterated mathētēs) of the apostles in chapter 11. This epistle is sometimes reckoned among the apostolic fathers, but I would prefer to count it as a Christian apology.
According to Bart Ehrman (on pages 127-129 of volume 2 of the Loeb edition of the apostolic fathers) and Michael Holmes (on page 294-295 of The Apostolic Fathers) there are no manuscripts extant for the epistle to Diognetus. The epistle was preserved in a single manuscript, dating to century XIII or XIV, which was discovered in 1436 in Constantinople. This manuscript, called Argentoratensis, passed through several hands before winding up at the municipal library in Strasbourg, where it was destroyed by fire during the Franco-German war on August 24, 1870. Fortunately, Henri Estienne, better known as Stephanus, had published the editio princeps of the epistle in 1592, based on his own transcription of 1586. B. Haus had produced another transcription in 1580, and J. J. Beurer yet another in 1590 (now lost, but included in the collation by Stephanus). Other editions followed that of Stephanus.