Through his father he was a descendant of Gregory the Illuminator, who converted the Kingdom of Armenia to Christianity in the early fourth century and became the first head of the Armenian Church.

[6] The Armenian historian Ghazar Parpetsi writes of him that he studied "among many learned Byzantines" and was "fully versed in musical notation, exhortatory rhetoric, and especially philosophy.

He supported King Khosrov IV of Armenia in his efforts to reunite the Armenian realm,[6] which had been partitioned into Roman and Sasanian parts in 387.

[6] Through Isaac's efforts the churches and monasteries destroyed by the Persians were rebuilt, education was cared for in a generous way, Zoroastrianism which Shah Yazdegerd I tried to set up was cast out, and three councils held to re-establish ecclesiastical discipline.

[11] Isaac encouraged Mesrop Mashtots to invent the Armenian alphabet and himself translated and revised parts of the Bible.

[3] According to Ghazar Parpetsi, Mashtots and the first translators frequently turned to Isaac, who had received a solid education, for assistance while making the alphabet.

Isaac had already established schools for higher education with the aid of disciples whom he had sent to study at Edessa, Melitene, Constantinople, and elsewhere.

[19] According to Abraham Terian, the identifiable works attributed to Isaac consist of canons and his two letters to Archbishop Proclus of Constantinople and Bishop Acacius of Melitene.

[20][a] Movses Khorenatsi's history quotes letters from Sahak to Eastern Roman emperor Theodosius II, the consul Anatolius, and Archbishop Atticus of Constantinople.

[2] In the Armenian Apostolic Church, his memorial day is marked twice a year: first, on the Saturday preceding the penultimate Sunday before Lent, thus falling between January 24 and February 28; second, together with Saint Mesrop Mashtots, on the Thursday following the fourth Sunday after Pentecost, thus falling between June 1 and July 16.

[15] The end of the first section of Ghazar Parpetsi's history contains a description of a vision supposedly seen by Sahak and narrated by him after he refused the request of the Armenian nobility to return to the patriarchal throne.

In the vision, Sahak saw various symbols connected with the Church and faith and saw that the Arsacids of Armenia and Gregorids would fall from power and apparently disappear; however, after a period of chaos both would be restored to their respective thrones; his daughter Sahakanoysh would give birth to "powerful and pious men.