Nephew of Patriarch Theophilus 
Cyril was born in the small town of Theodosios, Egypt. His mother's brother Theophilus had become the patriarch of Alexandria. He  was educated  and promoted Cyril and when he died in 412 Cyril became patriarch. He vigourously attacked Neoplatonism, put pressure on the civil authorities to expel the Jews and closed down  the churches and seized the sacred vessels of Novatianist schismatics. This led to tensions and violence in the city streets that resulted the murder of a Neoplatonist woman philosopher named Hypatia and a serious quarrel with the city prefect.

Confronting Nestorius
The power of his personality again came into play when the new patriarch of Constantinople Nestorius (428) began preaching against applying the title "theotokos" (= "bearer of God") in reference to Mary saying that it compromised Jesus humanity, insisting that "christotokos" ("bearer of Christ") was more appropriate.  Cyril's retort was that Nestorius was actually denying the reality of the Incarnation, by making Jesus Christ into two different persons, one human and one divine, in one body. He referred the matter to Pope Celestine I who charged him with seeing that Nestorius retracted.

The Emperor Theodosius II convened a council at Ephesus (431), a centre of Marian devotion where theotokos was popular. Two diametrically opposed points of view were at play here: the Antiochian catechetical school from which Nestorius came, emphasised a literal approach to Scripture and the human nature of Jesus, while the Alexandrian school, from which Cyril came, took an allegorical approach to Scripture and emphasised the divine nature.

Theotokos triumphant
The Emperor favoured the Nestorian view, while the Pope favoured Cyril's view. Cyril led the way at the Council, and  before John of Antioch and his bishops arrived he had condemned Nestorius who refused to plead before the council. John and his bishops attacked Cyril. The eventual outcome was that Nestorius was condemned as a heretic and deposed, even though there was a considerable minority that did not consent to the condemnation. The theotokos doctrine triumphed and the theme became central to Byzantine identity especially in the 6th century through the Akathistos (= "no sitting") Hymn in praise of Mary Mother of God. It was sung in the Temple of Santa Sophia when after a victory in war all the people gathered and stood singing. About Jesus the Council decreed that decreed that Jesus was one person, not two separate people: complete God and complete man, with a rational soul and body.

Legacy of bitterness
But there was a legacy of bitterness and not all the problems of christology were solved. Cyril felt the end justified the means and on balance his formula seems to have been proved right. But he made little effort to achieve a real meeting of minds.  The bishops who supported Nestorius were subsequently removed from their sees. And it took four years before the emperor Theodosius could bring himself to banish Nestorius to a monastery in Egypt. A separate Nestorian Church broke off at that time and still exists today in Iran, Iraq and Malabar, India.

John of Antioch eventually accepted the condemnation of Nestorius and was reconciled with Cyril. But there then appeared an over-emphasis on the divine nature (Monophysitism) which was not resolved until the Council of Chalcedon twenty years later (451). 

Cyril certainly had a clear grasp of the issues and continued to defend what he saw was the truth with energy and conviction. But he was impulsive, intransigent and somewhat triumphalist in his approach. He died in 444. Pope Leo XIII declared him a doctor of the church in 1883.