The Mar Saba letter is a Greek document which scholar Morton Smith reported in 1973 that he had discovered in the library of the Mar Saba monastery in 1958. The document has been lost and now only survives in two sets of photographs. The text purports to be an epistle of Clement of Alexandria and contains the only known references to a "Secret Gospel of Mark".
In 1973 Morton Smith published a book on a previously unknown letter of Clement of Alexandria. He stated that, while cataloging documents at the ancient monastery of Mar Saba in the summer 1958, he discovered the text of the letter handwritten into the endpapers of Isaac Vossius' 1646 printed edition of the works of Ignatius of Antioch. This letter is consequently referred to as the Mar Saba letter of Clement of Alexandria. Smith also published a second book for the popular audience in 1974.
Smith's books reproduced black-and-white photographs which he stated he had taken at the time of the discovery. In 1976 a group of four scholars visited Mar Saba, and viewed the manuscript. This visit remained unknown until 2003 when one of the party, G.A.G. Stroumsa, published an account of the visit. In 1977 the volume containing the manuscript was taken to the library of the Greek Orthodox Patriarchate in Jerusalem. That same year, the manuscript pages were removed from the bound volume by the librarian Kallistos Dourvas, to be photographed and kept separately. These photographs were published in 2000. Subsequent attempts by scholars to view the manuscript have been unsuccessful. Paleographers, working from Smith's photographs, have assigned dates from the late seventeenth to the early nineteenth centuries.
The letter, addressed to one Theodore, discusses a "Secret Gospel of Mark" and quotes two excerpts from this gospel, one of which mentions "the mystery of the kingdom of God." Clement begins by commending Theodore's actions against the Carpocratians, a heretical sect. He then turns to address questions posed by Theodore regarding the Gospel of Mark, a secret variant of which the Carpocratians claim to have. Clement admits to knowledge of a second secret or mystical version of the gospel, written by Mark for "those being perfected". However, he asserts that the version promoted by the Carpocratians is not an accurate representation of this; they have corrupted the original with their own false additions. To illustrate this, two ostensibly genuine excerpts from the gospel are supplied. The letter breaks off abruptly as Clement begins to explain the passages.