This extended work is intended to address the claims made by Theodore, the bishop of Mopsuestia, who allegedly was the counselor and defender of Nestorius, Archbishop of Constantinople. St. Cyril, along with many other of the church fathers, claim that the Nestorian heresy originates with Theodore and his claims about the divinity of Christ, and by extension, his mother the Theotokos. Theodore, during his lifetime, was regarded as an orthodox Christian churchman, who guarded against the Arianist camp, but was later condemned formally four years later at the Ecumenical Council of Ephesus. This work by St. Cyril is likely dated to that period between his death and the assembly of the council in Anatolia.