In what sense, then, does the fact that Jesus is truly God and truly man, relate to our salvation? Unlike his admired predecessor, St Athanasius, St. Cyril does not use the word ‘theosis’ very often, but, nonetheless, the concept of our divinisation is central to his thought; indeed, set in the Alexandrian tradition, and soaked in the writings of St. Athanasius the Apostolic, it would have been amazing had that not been the case. St. Cyril expands our understanding of the famous Athanasian saying that: ‘He was made man so that we might be made god.’
As relevant now, as then, was St. Cyril’s statement that someone who claimed to believe in God as a Christian must believe in God the Father, the Son who became Incarnate, and the Holy Spirit.  The Holy Spirit is fully part of the Godhead, since ‘all things are by the Father, through the Son in the Spirit’; this characteristically Cyrilline formula is one he refers to again and again.
If we look at St. John 17.23: ‘I in them and you in me, that they may become completely one’, we see his conception of the ‘economy’ that has taken place for our salvation. The Word leaves His equality with God the Father, emptying Himself, as in Philippians 2, and taking upon Himself an earthly temple from the Virgin’s womb, He became one with us also, but He was still what He had always been: Christ is one and the Son is one. Even though the flesh is not of the nature of the Father and does not enjoy union with Him, it is still one with the Word and is thus in union with God. In no other way can man have union with God except through the Incarnate Word. The union with the Spirit, was a union without confusion with God the Word and in an inexpressible way, sanctified the flesh; only this way can St. Athanasius’ saying be properly understood. Only through The Word’s own flesh can we come into contact with the Trinity; only through Eucharist, through the flesh of Christ, can we participate in His divinity.
For St. Cyril, John 1.14 is especially relevant here: ‘he says not that the Word came into flesh, but that It was made Flesh, that you might not suppose that He came into it as in the case of the prophets or any other of the Saints by participation, but did Himself become actual Flesh, that is man’. [In Jo. 1:14.]
This anti-docetic emphasis points us to the crux of his future disagreement with Nestorius, for it develops St. Athanasius’ soteriology; only through the Incarnate Word is God is able to lead humanity to deification. Both Saints are following the Pauline theme, developed in Philippians, of divine kenosis. The union of the divine and the human allows the sanctification and deification of humanity; through that we are united with the Father. Long before Nestorius preached, St, Cyril was teaching the truth expressed in St. Gregory of Nazianzus’ phrase: ‘what is not assumed cannot be healed.’ There are, as he shows us in his commentary on John 6:22, two stages in our sonship: the first, through the Incarnation, which is a sonship in general; the second comes in our personal participation in the divine nature through the Holy Spirit and the Eucharist.
For St. Cyril the renewal of mankind is not simply a return of man to his original state; like St. Irenaeus before him, he maintained that Christ ‘did not simply return man to his original state, but offers him gifts from God which were not in the possession of Adam.’  Adam did not partake of the divine nature; only in Christ did man receive divine sonship. Through the ‘second Adam’ mankind gains far than it had lost in the Fall: ‘we became diseased through the disobedience of the first Adam and his curse, but we have become rich through the obedience of the second and his blessings.’ 
Mankind cannot grasp this blessing by its own efforts; only through the mediation of the Spirit, in the sacraments, can we become sons of God. It is in St. Cyril that the concept of divinisation as taught by the earlier Fathers reaches full maturity.  :
‘God the Father therefore gives life to all things through the Son in the Holy Spirit’,  and the Son, by putting on our nature, refashions it to his own life. And he himself is also in us, for we have all become partakers of him, and have him in ourselves through the Spirit. For this reason, we have become ‘partakers of the divine nature’ (2 Peter 1:4), and are reckoned as sons, and so too have in ourselves the Father himself through the Son.’ [In Jo. 14.20]
Salvation is the work of the whole Trinity, not of one part of it.
 St. Athanasius, De Incarnatione 54.
 Farag, 78 for full references.
 R.L. Wilken, Judaism and the Christian Mind (NY, 1971)
 Koen, p. 41, citing In Jo. 1:14.
 Keating, Theology of St. Cyril, p. 149.
 St. Cyril of Alexandria, Commentary on the Gospel of St. Luke, part 1 (Oriental Orthodox Library, 2006, In Luc. 22:17-22 p. 569