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.”