The treatise About the Mystery of the Letters is an ancient book dated to the second half of the sixth century which is preserved in Coptic and Greek. The author is believed to be „among the monks of a Sabaite community, possibly the most important among them, the Great Laura Mar Saba near Jerusalem, which still exists today.“ 
The work explores „the secret messages of the Greek alphabet.“ Its content is built on the basic idea „that there is an original connection between writing and creation.“ And „that not only in the form and shape of the letters is the mystery of Christ revealed, but also in their names and their namings.“ Thus, the author writes, „A mystery of God it is that is in the letters of the alphabet.“ And that Christ says, „I am the Alpha and the O. That is, He who is described by the whole alphabet.“
The Greek title of the treatise is:
Περὶ τοῦ μυστηρίου τῶν γραμμάτων
Peri tou mysteriou ton grammaton
About the mystery of the letters
At once I understood: It is a mystery of God which is <contained> in the letters of the alphabet, and one which he has not revealed to us!
Tractate About the Mystery of the Letters, Introduction, 21-23.
The Beinecke Rare Books & Manuscript Library at the Yale University (USA) holds a heretofore unknown version of the Treatise on the Mystery of Letters copied in 1886. The manuscript with the inventory number „Coptic MS 13“ is the first and so far only known monolingual Arabic version of the Treatise. The library wrote an online article about the manuscript, including beautiful images of the drawings of individual letters: https://beinecke.library.yale.edu/article/beinecke-coptic-ms-13
 Cordula Bandt, Der Traktat „Vom Mysterium der Buchstaben“ (Berlin 2007), (n. 33), 7.
 Bandt (n. 33) 1.
 Bandt (n. 33) 2.
 Mysterium der Buchstaben, Kap. 23.12-14, Übersetzung Bandt (n. 33) 157.
 Mysterium der Buchstaben, Vorrede, 21-22 (koptischer Teil), Übersetzung Bandt (n. 33) 103.
 Mysterium der Buchstaben, Kap. 36.12-13, Übersetzung Bandt (n. 33) 159.