No brief survey can do justice to St. Cyril’s multiple contributions to our understanding of the Faith ‘once received’: his Trinitarianism and his Christology are the very summit of the achievement of the Eastern Fathers. His debt to Origen, St. Athanasius and to the Cappadocians, as well as to St. Irenaeus is obvious, but he brought their work to a new perfection. If the Western traditions have not always given him the credit that is his due – and his absence from the standard edition of the Church Fathers is much to its detriment `- then he has remained a powerful influence on the Oriental Orthodox tradition, not least in his own Coptic Church. A true Christology has to be related to a true soteriology, one that really transforms mankind and raises us to life in God.
This was why, when St. Cyril heard that Nestorius was speaking of the ‘two natures’ of Christ, he became concerned. He told bishop Succenus that because Nestorius ‘isolates the individual man born of the holy Virgin and likewise the individual Son, the Word from God the Father’, he ‘declares the holy Virgin is not the mother of God but mother of the man.’  The correct doctrine is that Christ is the pre-eternal Word born of the Virgin. St. Cyril knew that some were accusing him of an Appolinarian understanding of the Incarnation, and thought he was teaching a merger or a mingling of the two natures. This he dismissed as a ‘slander’, asserting what his own Church has ever held:
We affirm that the Word from God the Father united to himself in some inscrutable and ineffable manner, a body endowed with mental life and that he came forth, man from woman, become what we are, not by change of nature but in gracious fulfilment of God’s plan. In willing to become man he did not abandon his being God by nature; though he descended to our limited level and worse the form of a slave, even in that state he remained in the transcendent realms of Godhead and in the Lordship belonging to his nature.
So we unite the Word from God the Father without merger, alteration or change to holy flesh owning mental life in a manner inexpressible and surpassing understanding, and confess one Son, Christ and Lord, the self-same God and man, not a diverse pair but one and the same, being and being seen to be both things. 
There is ‘one incarnate nature of the Word’, and after union, there should be no speaking of two natures.
St. Cyril has been criticised for his use of the phrase ‘the one incarnate nature of God the Word’, and some hold that he was ‘taken in’ by an Appolinarian forgery which he thought Athansian in origin. A full discussion of this topic lies beyond the scope of this paper,  but this does him a serious injustice. As he wrote to his agent in Constantinople. Eulogius: ‘there is no obligation to reject everything heretics say – they affirm many of the points we too affirm.  Apollinarius had come to the wrong conclusion, but he had identified the need for the Church to confess a single subject in the Incarnate Word. This had been at the heart of Alexandrian theology from Origen’s day, and has led even recent scholars to assert that ‘a single subject Christ, with an emphasis on Christ’s divinity’ was part of the Alexandrian tradition.  But this is to misread things. St. Cyril’s soteriology was a dynamic one, in which, as we have seen, enfleshment and the Logos were both essential parts of the Cyrilline vision.