Source: A portrait at Dayro d-Mor Ephrem, Glane

Known as "Moses of Mesopotamia", Mor Ya`qub was the bishop of Nisibin from 308/9 A.D. till his death in AD 338. Mor Ya`qub has always been a prominent figure in the Syriac-speaking Church tradition. He acquired a reputation for great learning, ability and holiness. Little is known about his life; however, we know that he took a leading role in opposing the Arian heresy at the Council of Nicaea (325). He is honored as a malphono (i.e., theological doctor) by both the Syriac and Armenian Churches. Mor Ya`qub of Nisibin undertook the construction of a church building in Nisibin between 313 and 320. His relics were saved from the a Persian invasion and were send to Constantinople for safety around the year 970.

It was under the bishopric of Mor Ya`qub that Mor Ephrem flourished. The poet saint speaks of his bishop Mor Ya`qub,

By his simple words he gave milk to his infants.
The Nisibin Church was childlike with him.
As with a child, he loved her and threatened her.
The womb of him who gave birth to the flock bore her infancy.
The first priest gave milk to her infancy.
The wealthy father, laid up treasures for her childhood.
(Hymns on Nisibin 14.16-22)

His memory is commemorated on May 12.

Source:

The Oxford Dictionary of the Christian Church, 3rd ed. (1997).

Kathleen McVey, Ephrem the Syrian Hymns (Paulist Press, 1989).