Most likely, Cyril was born between 370 and 380 in Alexandria, the Egyptian capital. It seems from his writings that he had a strong religious and literary education. He was involved in a conflict between the Greek and Egyptian churches at the beginning of the fifth century, along with his uncle, Patriarch Theophilus of Alexandria. Evidence suggests that he was a monk before rising to the position of bishop.

Cyril was selected to take Theophilus' place as the leader of the Egyptian Church after he died in 412. Despite Constantinople's political dominance, he carried on his uncle's goal of upholding Alexandria's superiority within the Church. In about 418, the two Eastern churches reestablished communion.

However, a theological disagreement ten years later led to another rift between Alexandria and Constantinople. Cyril's defense of Catholic orthodoxy at this period helped to establish his reputation as a theologian and, eventually, as a Doctor of the Church.

Nestorius, a monk, succeeded as the new patriarch of Constantinople in 428. It became evident that Nestorius was unwilling to refer to the Virgin Mary as the "Mother of God" (or "Theotokos"). Rather, he was adamant about using the title "Mother of Christ" (Christotokos).

The Greek Church had already convened two ecumenical councils in the fourth century to affirm the everlasting preexistence of Christ as God before his incarnation as a man. It was a natural deduction from this enduring conviction that Mary was God's mother. The incarnation doctrine and Christ's equality with God the Father were validated by Mary's veneration as "Theotokos."

Nestorius asserted that these were his own beliefs. However, Cyril and several others perceived his reluctance to see Mary as the Mother of God as an indication of a heretical perspective on Christ that would divide him into two unified yet separate individuals: one who was entirely human and descended from Mary, and the other who was altogether divine and not susceptible to birth or death.

In response to this heretical trend, Cyril first wrote several letters to Nestorius, then he appealed to the Pope, and in 431 he called an ecumenical council. In his presidency, Cyril declared that he was "filling the place of the most holy and blessed Archbishop of the Roman Church," Pope Celestine, who had given the council official authorization.

The council met in turbulent circumstances. Nestorius's companion, Patriarch John of Antioch, visited the city and called a rival council intending to impeach and denounce Cyril. Both sides tried to persuade the emperor in Constantinople to support their positions, and occasionally the animosity between the supporters of Cyril and Nestorius descended into physical bloodshed.

At the council, which took place in 431 AD between June 22 and July 31, Cyril skillfully upheld the traditional view of Christ as a single, everlastingly divine being who also took on human form. Nestorius was banished and had his position as patriarch removed after the council's condemnation. On the other hand, Cyril made amends with John and a large number of other Antiochian theologians who had previously backed Nestorius.

On June 27, 444, St. Cyril of Alexandria passed away. He had served as a bishop for about 32 years. He was named a Doctor of the Church in 1883 after being revered as a saint for a long time, especially in the Eastern Catholic and Eastern Orthodox churches.

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