During the first Christian centuries the school of Nisibis was the spiritiual center of the Assyrian Church of the East.
The school was founded around 350 by Mar Jacob after the model of the school of Diodorus of Tarsus in Antioch. It was the perfect location for a Syriac school: located in the center of the Syriac speaking world, and still inside the Roman empire, which had just embraced Christianity. Most of Mesopotamia was under Sassanidian Persian rule, which at that time tried to revive the Zoroastrian religion.
Exile to Edessa
The Persians gained Nisibis already in 363 and the school was moved westward to Edessa, where it was known as the 'school of the Persians'. There, under the leadership of Ephrem the Syrian, it gained fame well beyond the border of the Syriac speaking world.
Meanwhile in Antioch Theodore of Mopsuestia had taken over the school of Diodorus, and his writings soon became the foundation of Assyrian theology. Even during his lifetime they were translated into Syriac and gradually replaced the work of Ephrem.
During the Nestorian schism the opponents of Nestorius attacked Diodore as well, and the Syrians answered by giving protection to the followers of Nestorius. In 489 the Byzantine emperor ordered the school closed for its Nestorian tendencies and it returned to Nisibis.
Center of Assyrian theology
Back in Nisibis the school became even more famous. It attracted students from all the Assyrian Church, many of its students embodied important church offices, and its teaching was normative. The exegetical methods of the school followed the tradition of Antioch: strictly literal, controlled by pure grammatical-historical analysis. The work of Theodore was central to the theological teaching, and men like Abraham of Beth Rabban, who headed the school during the middle of the 6th century, spend great effort to make his work as accessible as possible. The writings of Nestorius himself were added to the curriculum only about 530.
At the end of the 6th century the school went through a theological crisis when its director Henana of Adiabene tried to replace Theodore by his own doctrine, which followed Origen. Babai the Great (551-628), who was the inofficial head of the Church at that time and also involved in reviving the strict Assyrian monastic movement, refuted him and in the process wrote the normative Christology of the Assyrian Church, based on Theodore of Mopsuestia.
A small sampling of Babai's work is available in English translation here (http://www.cired.org/faith/bawai.html). The Book of Union is his principle surviving work on Christology. In it he explains that Christ has two qnome (essences), which are unmingled and eternally united in one parsopa (personality). This, and not Nestorianism, is the teaching of the Assyrian Church.
Influence on the West
The fame of this theological seminary was so great that Pope Agapetus I and Cassiodorus wished to found one in Italy of a similar kind. The troubled times prevented their wishes from being realized, but Cassiodorus's monastery at Vivarium was inspired by the example of Nisibis.