The Letter (or Epistle) to Diognetus is a second- or third-century work that defends the doctrines of Christianity. This type of writing is also called an apologetics letter or an apology. Although neither author nor recipient is known for certain, the Letter to Diognetus purports to be written by a student of the apostles who calls himself Mathetes, which in Greek simply means “disciple.” The identity of the Diognetus addressed in this letter is uncertain, although he may have been a man of some rank, as the salutation addresses him as “His Excellency.”
The epistle contains 12 chapters describing the meaning and results of salvation by faith in Christ, although the name Jesus is never mentioned. References to “the Son” or “the Word” take the place of the name Jesus Christ. The oldest known manuscript of the Letter to Diognetus, dating from the thirteenth or fourteenth century, was found along with the writings of Justin Martyr; unfortunately, that manuscript was destroyed in a fire in 1870. For a time it was believed that the Letter to Diognetus was composed by Justin Martyr, but that theory has since been discarded.
In this Letter to Diognetus, the author describes the difference between a Christian and a non-Christian. He also contrasts the Jewish religion with the fulfillment of that religion in Christianity and calls the Jews “foolish” who continue to cling to the letter of the Law rather than live in the freedom purchased by the Son of God. The end of Chapter 5 contains an apt description of the life Jesus has called His followers to live:
“They exist in the flesh, but they do not live by the flesh. They pass their days on earth, but they are citizens of heaven. They obey the prescribed laws, all the while surpassing the laws by their lives.
“They love all men and are persecuted by all. They are unknown and condemned. They are put to death and restored to life.
“They are poor, yet make many rich. They lack everything, yet they overflow in everything.
“They are dishonored, and yet in their very dishonor they are glorified; they are spoken ill of and yet are justified; they are reviled but bless; they are insulted and repay the insult with honor; they do good, yet are punished as evildoers; when punished, they rejoice as if raised from the dead. They are assailed by the Jews as barbarians; they are persecuted by the Greeks; yet those who hate them are unable to give any reason for their hatred.”
One truth magnified in the Letter to Diognetus is that the lifestyle Jesus called all Christians to live has not changed. Unwarranted persecution has existed since Jesus walked the earth and will continue until He comes again (Matthew 5:10–12; John 15:18–19). The Letter to Diognetus is a clear and succinct explanation of what it means to follow Jesus and that there is no other life worth living (see Luke 9:23; John 6:68).