Agathangelos (in Old Armenian: Ագաթանգեղոս Agatʿangełos, in Greek Ἀγαθάγγελος "bearer of good news" or angel), appropriately so named, was a supposed secretary of Tiridates III, King of Armenia, under whose name there has come down a life of the first apostle of Armenia, Gregory the Illuminator, who died about 332. It purports to exhibit the deeds and discourses of Gregory, and has reached us in Armenian and in Greek. The text of this history has been considerably altered, but it has always been in high favor with the Armenians. Von Gutschmid maintains that the unknown author made use of a genuine life of St. Gregory, also of a history of his martyrdom and of that of St. Ripsime and her companions. Historical facts are intermingled in this life with legendary or uncertain additions, and the whole is woven into a certain unity by the narrator, who may have assumed his significant name from his quality of narrator of "the good news" of Armenia's conversion. It has been translated into several languages, and Greek and Latin translations are found in the Acta Sanctorum Bollandistarum, tome viii.
According to Agathangelos, he was tasked by Trdat III Armenian king to write about Trdat's father Khosrov and his reign period. Until 19th century, based on this fact, scholars believe that Agatangelos lived in 4th century. However further detailed research of his writing has demonstrated that Agathangelos lived and worked in 5th century and was not able to be Trdat's secretary.
See: Agathangelos, History of the Armenians, Robert W. Thomson, State University of New York Press, 1974