The Teaching of Addai is a Syriac document convincingly dated by some scholars in the fourth or fifth century AD. There may be some points containing possible historical traces that go back even to the first century AD, such as the letters exchanged by king Abgar and Tiberius. Some elements in them point to the real historical context of Abgar 'the Black''s reign in the first century. The author of the Doctrina might have known the tradition of some historical letters written by Abgar and Tiberius.
Extant in mss of the fifth-sixth cent. AD: Brit. Mus. 935 Add. 14654 and 936 Add. 14644. Ed. W. Cureton, Ancient Syriac Documents (London 1864; Piscataway: Gorgias, 2004 repr.), 5-23; another ms. of the sixth cent. was edited by G. Phillips, The Doctrine of Addai, the Apostle (London, 1876); G. Howard (tr.), The Teaching of Addai, SBL Texts and Translations, 16, Early Christian Literature Series, 4 (Chico: Scholars Press, 1981), with Phillips' edition and a new English version; R. Peppermueller, "Griechische Papyrusfragmente der Doctrina Addai" (VChr 25 [1971]), 289-301; A. Desreumaux, "La Doctrine d'Addaï" (Aug. 23 [1983]), 181-86; Id., Histoire du roi Abgar et de Jésus (Turnhout: Brepols, 1993). On the Abgar legend see H.J.W. Drijvers, "The Abgar Legend," in New Testament Apocrypha, ed. W. Schneemelcher, trans. R. McL. Wilson (Louisville: John Knox, 1991), 492-99. A datation about AD 400 is often found in encyclopaedia articles, such as P. Bruns, "Addai (Doctrina Addai)," in Lexikon der antiken Christlichen Literatur, Hrsg. S. Döpp - W. Geerlings (Freiburg-Basel-Wien, 2002), 7, and C.&F. Jullien, Apôtres des confins, Res Orientales 15 (Louvain: Peeters, 2002), 67ff. The Addai story is also known in Greek, Armenian, Coptic, Ethiopic, Arabic, Georgian, and Slavonic.
L.-J. Tixeront, Les origines de l'église d'Édesse et la légende d'Abgar (Paris: Maisonneuve, 1888), fixed the definitive redaction of the Doctrina to AD 390-430; Acta Apostolorum Apocrypha, edd. R.A. Lipsius - M. Bonnet, I (Lipsiae 1891, repr. Hildesheim: Olms, 1990), CIXff.; 279-83, give 360-90; see also L. Moraldi, Gli apocrifi del Nuovo Testamento, II (Torino: UTET, 1971), 1647. On the evangelization of Mesopotamia: Jullien, Apôtres; W. Baum - D.W.
Winkler, The Church of the East. A Concise History (London: Routledge, 2003).
Phillips G. The doctrine of Addai, the Apostle. London, 1876.
http://ia310831.us.archive.org/attachpdf.php?file=%2F1%2Fitems%2Fdoctrineofaddaia00phil%2Fdoctrineofaddaia00phil.pdf