Today we meet an important figure: St Cyril of Alexandria (370/80-444). In the Greek Orient, Cyril was later defined as “the guardian of exactitude” —to be understood as guardian of the true faith— and even the “seal of the Fathers” (because of his constant reference to earlier ecclesiastical authors —including, in particular, Athanasius— for the purpose of showing the continuity with tradition of theology itself, recognized as guaranteeing continuity with the Apostles and with Christ himself).

Cyril was initiated into ecclesiastical life while he was still very young and received a good education, both culturally and theologically. Upon his uncle Theophilus' death (the bishop of Alexandria), the still young Cyril was elected in 412 as Bishop of the influential Church of Alexandria, which he governed energetically for 32 years.

When in 428 Nestorius was elected, the new Bishop of Constantinople, in fact, soon provoked opposition because he preferred to use as Mary's title in his preaching “Mother of Christ” instead of “Mother of God”, already very dear to popular devotion. One reason for Bishop Nestorius' decision was that in order to safeguard the importance of Christ's humanity, ended by affirming the division of the Divinity. The reaction of Cyril was almost immediate (cf. Council held in Ephesus, in 431).

— St Cyril of Alexandria was an unflagging, staunch witness of Jesus Christ, the Incarnate Word of God, emphasizing above all his unity: «Only one is the Son, only one the Lord Jesus Christ, both before the Incarnation and after the Incarnation».