Arsacid dynasty of Armenia
Isaac was son of the Christian St. Nerses I and a Mamikonian princess called Sanducht. Through his father he was a Gregorid and was descended from the family of St. Gregory I the Enlightener. He was the fifth Catholicos of the Arsacid dynasty of Armenia after St. Gregory I the Enlightener (301–325), St. Aristaces I (325–333), St. Vrtanes I (333–341) and St. Husik I (341–347). His paternal grandmother was the Arsacid Princess Bambish, the sister of King Tigranes VII (Tiran) and a daughter of King Khosrov III.