From the longer Wikipedia page 
The Mar Saba Letter is an epistle attributed to Clement of Alexandria and discovered by Morton Smith (Wikipedia page ) in 1958. It contains the only known references to the 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 he claimed to have 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.
Scholars Philip Jenkins and Robert M. Price noticed parallels between The Secret Gospel of Mark and a novel by James Hunter published in 1940 entitled The Mystery of Mar Saba. In 1980 the Mar Saba letter was included in the revision of the standard edition of works of Clement of Alexandria: Otto Stählin and Ursula Treu, Clemens Alexandrinus, vol. 4.1: Register, 2nd ed. (Berlin:Akademie-Verlag, 1980), XVII–XVIII.
Nevertheless, doubts have been expressed about its authenticity. In a 1975 review of Smith's book, Quentin Quesnell raised doubts about the original manuscript and suggested that it was a forgery executed sometime between 1936 and 1958. Though Quesnell did not specifically accuse Smith, in the view of Charles W. Hedrick he "broadly hinted" that Smith was the culprit. When Quesnell wrote this, no scholar other than Smith had claimed to have seen the manuscript.
In 2005 Stephen Carlson published Gospel Hoax: Morton Smith's Invention of Secret Mark, which also asserted that the manuscript was a hoax. By this time independent confirmation of the manuscript's existence and appearance had been presented by Hedrick and others; Carlson argued that Smith had himself written the text into the book.
Earlier in the same year, Scott Brown published Mark's Other Gospel: Rethinking Morton Smith's Controversial Discovery. In this book he claimed that the Secret Gospel of Mark was an authentic writing of the evangelist, implying that the letter was authentic.
Many scholars who accept the letter as a copy of an ancient manuscript believe that it is not the work of the historical Clement. There seems to have been another pseudo-Clement, who was mentioned in the Decretum Gelasianum as "the other Clement of Alexandria." The central element of initiation and progress to "the innermost sanctuary of that truth hidden by seven veils" is common to Gnostic writings and to the mystery religions of the period. Even so, Clement believed Christianity to be the pure representative of God's true Mysteries which others had stolen and corrupted.