Mar Jacob of Nisibis is commemorated on the first Friday of the period of Summer. He is called the "Moses of Mesopotamia" for his wisdom and wonderworking abilities, was the second bishop of Nisibis, spiritual father of the renowned Syriac writer and theologian Ephrem the Syrian, celebrated ascetic and one of the 318 fathers of the First Ecumenical Council at Nicaea.
At an early age he devoted himself to the life of a solitary, practicing the severest self-discipline. He liked the solitude and the peace of the desert, and he lived in the mountains around the city of Nisibis, on the border of the Persian and Roman empires. In the summer he lived in crevices of the mountains, and in the winter he lived for a short time in a cave. His food was not what he had sown, but what grew there on its own, such as fruits from wild trees and green plants that grew in the desert. His clothing was made of hard goat's hair.
He always fed on spiritual food which came through prayer which also kept his thoughts pure. Through his asceticism, he gained a deeper connection with God. He had the gift of foresight, and by the grace of the Spirit, he received the gift of miracles.
In 325 Jacob was summoned to the Council of Nicaea. A leading part is ascribed to him by Theodoret in the debates of that council, as the champion of the whole Orthodox faith.
Around A.D. 350 Mar Jacob founded the School of Nisibis, after the model of the school of Diodorus of Tarsus in Antioch, in which he himself was an instructor. The venerable Mar Jacob died peaceably around A.D. 350.
Text exerpt provided by OrthodoxWiki.