At the beginning of the controversy between St Cyril of Alexandria and Nestorius, the latter had preached a sermon arguing against the use of the term Theotokos (Θεοτόκος, the Mother of God) based on his understanding of who Jesus Christ was. Cyril corresponded with Nestorius, who was the Patriarch of Constantinople, to correct him. Nestorius refused to be corrected; Cyril eventually wrote 12 anathemas which were adopted by the Third Ecumenical Council (Ephesus, 431), and Nestorius was condemned.
Why is this important? For many reasons, but ultimately, it clarified who Jesus, the Son of God, the Logos of God is. Whom do we worship?
Out understanding of who Jesus Christ is, the Divine Logos who unites in his Person human nature and divine nature, bears direct relevance to our understand of how we can be saved, how we can be healed and deified.
These issues were clarified even further by the Council of Chalcedon (451).
Here are the relevant excerpts from Nestorius’ arguments and Cyril’s response. Cyril’s original texts and the Greek technical terms which later became standard in Christian theology and Christology are provided in the footnotes.
Nestorius’ First Sermon Against the Theotokos
Nestorius asserts that “humanity is the image of the divine nature” and so “God fashioned from the Virgin a nature” and so by that “human being” he revived the human race. In this way, the Virgin is not Θεοτόκος, but ανθροποτόκος, the Mother of a man and not of God. Nestorius’ emphasis on nature rather than on the person becomes evident as he argues that God cannot have a Mother (idea which, according to him, would be Greek paganism), and that Mary did not give birth to the “Godhead” (emphasis on nature) but to a “human being;’ the “incarnate God did not die, he raised up the one in whom he was incarnate.” It is clear that Nestorius has at least two individuals in mind, the “incarnate God” who cannot be born and die, and the “one” who is born, dies, and is raised by the incarnate God.
For him, “Christ Jesus” (e.g., as mentioned by Paul in Phil. 2) is not the same as God the Logos; in some fashion, he argues, “Christ is an expression of the two natures, and “Christ assumed the person of the debt-ridden nature.” He “assumed a person” of the same nature as ours, he “assumed man” and “the third-day burial belonged to this man, not the deity.”
Nestorius ends with two persons, which raises the question of whom exactly he worships. He answers, “I worship this one [the man] together with the Godhead.” He says, “I revere the one who is borne [the man] because of the one who carries him [the Logos], and I worship the one I see because of the one who is hidden . . . I divide the natures, but I unite the worship” because “that which is formed in the womb is not in itself God . . . that which is buried in the tomb is not itself God. If that were the case, we should manifestly be worshippers of a human being and worshippers of the dead. But since God is within the one who is assumed,” the latter is “styled God” even though God “has not shared its suffering.” This man is “joined to omnipotent deity.”
Cyril of Alexandria, Second Letter to Nestorius
Cyril’s response makes clear that, contrary to what Nestorius says, Nestorius truly worships a man when he fails to understand that we worship God the Logos who assumed a human nature hypostatically. It is by worshipping the God who was born, died and rose again that we worship God the Logos.
Cyril begins by emphasizing that the Niceno-Constantinopolitan creed stated that it was the only begotten Son himself who was born according to nature of God the Father . . . he came down, and was incarnate, and was made man, suffered, and rose again the third day, and ascended into heaven.
Cyril emphasizes that the Logos from God took flesh and became human: “rather we say that the Logos having personally [hypostatically] united to himself flesh animated by a rational soul, did in an ineffable and inconceivable manner become man, and was called the Son of Man, not merely as willing or being pleased to be so called, neither on account of taking to himself a person but because the two natures being brought together in a true union there is of both one Christ and one Son.
The Logos was born of a Woman after he had united humanity hypostatically to himself. It is not that first a man was born anew from the Holy Virgin and later the Logos descended upon him. Rather, the union being made in the womb itself, he is said to undergo a fleshly birth, ascribing to himself the birth of his own flesh.
Cyril emphasizes against Nestorius that “we confess one Christ and Lord, and we do not worship a man along with the Logos, but we worship one and the same because there are not two Sons enthroned together but One because of the His own union with his own flesh.
Therefore, the Fathers boldly believe that Holy Virgin is the Θεότοκος (Theotokos, the Mother of God) not because the divine nature of the Logos began to exist in her, but the body with the rational soul, which was born, was hypostatically united to the Logos.
On the Anathemas and Worship:
The Anathemas promulgated by Cyril address the issue of worship both directly and indirectly. Directly, anathema VIII explicitly addresses the issue:
VIII. If anyone dares to maintain that the ascended man ought to be worshipped together with the divine Word, and be glorified with Him, and with Him be called “God” as one with another [ὡς ἕτερον ἑτέρῳ] (the addition of “along with” [σύν] will always entail this interpretation), and does not instead honor in one act of worship Emmanuel and praise Him in one doxology, in that He is the Word made flesh, let him be anathema.
In other words, separating a man from the Logos inescapably results in worshipping a man, distinct from the Logos, along with the Logos.
Similarly, other anathemas touch on the issue indirectly. For example,
V. If anyone dares to maintain that the Christ is a man bearing God [θεοφόρον ἄνθρωπον], and not rather that He is God in truth, and single Son by nature [φύσει], according as the Word was made flesh, and shared blood and flesh in like manner with ourselves, let him be anathema.
Or, if Christ is not the same Person as the Logos, then one would worship a man (Christ) along with God (the Logos); furthermore,
VII. If anyone says that Jesus was energized as man by God the Word, and that He was invested with the glory of the only-begotten as being another beside Him [ὡς ἑτέρῳ παρ᾿ αὐτὸν], let him be anathema.
Here Cyril uses language that includes adoptionist ideas: if Jesus is “energized” by the Logos (άνθρωπον ενηργήσθαι παρα του Θεου Λογου τον Ιησουν), then there are two persons, Jesus and the Logos, and a man and God are both worshipped.
XI. If anyone does not confess that the Lord’s flesh is life-giving [ζωοποιὸν], and proper to the Word of God Himself [ἰδίαν αὐτοῦ], but (states) that it is of another than Him, united indeed to Him in dignity, yet as only possessing a divine indwelling, instead of being lifegiving, because it is proper to the Word [ἰδία τοῦ λόγου] of Him who has the power to give life to all, let him be anathema.
That is, if the flesh of the Lord is not the flesh of God the Logos, but of a man who received the Logos, this flesh is not lifegiving; one would worship the flesh of a man, rather than God.
The birth, death and resurrection of a man would have been just that – what happened to that man. However, it is the Incarnation which heals and saves us. It is the birth of the Logos who takes flesh and unites human nature to himself; it is the life of God, the death of God, the burial of God and the resurection of God the Logos that deifies us.
 Αυτόν τον εκ Θεού Πατρός κατά φύσιν γεννηθέντα Υιόν μονογενή . . . κατελθείν, σαρκωθήναι τε και ενανθρωπήσαι, παθείν, αναστήναι τη τρίτη ημέρα, και ανελθείν εις ουρανούς.
 Εκείνο δε μάλλον ότι σάρκα εμψυχωμένην ψυχή λογική ενώσας ο Λόγος εαυτώ καθ’υπόστασιν, αφράστως τε και απερινοήτως γέγονεν άνθρωπος, και κεχρημάτικες Υιός ανθρώπου.
 Αλλ’ ουδε ώς εν προσλήψει προσώπου μόνου.
 Προς ενότητα την αληθινήν συναχθείσαι φύσεις.
 Είς δε εξ αμφοτέρων Χριστός και Υιός.
 Ενώσας εαυτό καθ’υπόστασιν το ανθρώπινον, προήλθεν εκ γυναικός, ταύτη τοι λέγεται γεννηθήναι σαρκικώς.
 Ου γαρ πρώτον άνθρωπος εγεννήθη κοινός εκ της αγίας Παρθένου, είθ’ ούτω καταπεφοίτηκεν επ’ αυτόν ο Λόγος.
 Αλλ΄εξ αυτής μήτρας ενωθείς, υπομείναι λέγεται γέννησιν σαρκικήν, ώς της ιδίας σαρκός την γέννησιν οικειούμενος.
 Ούτω Χριστον ένα και Κύριον ομολογήσομεν· ουχ ως άνθρωπον συμπροσκυνούντες το Λόγω
 ώς ένα και τον αυτόν προσκυνούντες
 Ουχ ώς δύο πάλιν συνεδρευόντων υιών
 Αλλ’ ώς ενός καθ’ ένωσιν μετά της ιδίας σαρκός
 Ου διαιρετέον τοιγαρούν εις υιούς δύο, τον ένα Κύριον Ιησούν Χριστόν.
 Ου γαρ είρηκεν η Γραφή ότι ο Λόγος ανθρώπου πρόσωπον ήνωσεν εαυτώ, αλλ’ ότι γέγονε σάρξ.
 Oύτω τεθαρσήκασι [οι Πατέρες] Θεοτόκον ειπείν την αγίαν Παρθένον
 Oυχ ώς της του Λόγου φύσεως
 Αλλ’ ώς γεννηθέντος εξ αυτής του αγίου σώματος, ψυχωθέντος τε λογικώς, ώ και καθ’ υπόστασιν ενωθείς ο Λόγος, γεγεννήσθαι λέγεται κατά σάρκα.