A similar attempt to reduce to a logomachy the difference between the Catholic faith and Nestorianism has been made by some writers of undoubted learning among Protestants, notably by Fuchs and Schröckh. But as in the case of the homousios so, too, in the case of the theotocos the word expresses a great, necessary, and fundamental doctrine of the Catholic faith. It is not a matter of words, but of things, and the mind most unskilled in theology cannot fail to grasp the enormous difference there is between affirming, as does Nestorianism, that a God indwelt a man with a human personality of his own distinct from the personality of the indwelling god; and that God assumed to himself human nature, that is a human body and a human soul, but without human personality.
(Wm. Bright, St. Leo on the Incarnation, pp.160, 161.)
It is, then, clear that the question raised by the wide circulation of the discourses of Nestorius as archbishop of Constantinople was not verbal, but vital. Much of his language was irrelevant, and indicated some confusedness of thought: much would, of itself, admit of an orthodox construction; in one of the latest of his sermons, which Garnier dates on Sunday, December 14, 430, he grants that "Theotocos" might be used as signifying that "the temple which was formed in Mary by the Holy Spirit was united to the Godhead;" but it was impossible not to ask whether by "the temple" he meant the body of Jesus, or Jesus himself regarded as a human individual existing idia, idikos, ana meros -- as Cyril represents his theory -- and whether by "union" he meant more than a close alliance, ejusdem generis, in the last analysis, with the relation between God and every saint, or, indeed, every Christian in true moral fellowship with him -- an alliance which would amount, in Cyril's phrase, to no more than a "relative union," and would reduce the Saviour to a "Theophoros," the title claimed of old by one of his chief martyrs. And the real identity of Nestorius's view with that of Theodore [of Mopsuestia] was but too plainly exhibited by such statements as occur in some of the extracts preserved in Cyril's treatise Against Nestorius -- to the effect that Christ was one with the Word by participation in dignity; that "the man" was partaker of Divine power, and in that sense not mere man; that he was adored together with the Word; and that "My Lord and my God" was a doxology to the Father; and above all, by the words spoken at Ephesus, "I can never allow that a child of three months old was God."
It is no part of my duty to defend the truth of either the Catholic or Nestorian proposition -- each has found many adherents in most ages since it was first started, and probably what is virtually Nestorianism is to-day far more widely held among persons deemed to be orthodox than is commonly supposed. Be this as it may, Nestorianism is clearly subversive of the whole Catholic Doctrine of the Incarnation, and therefore the importance of the word Theotokos cannot be exaggerated.
I shall treat the word Theotocos under two heads; (1) Its history (2) its meaning, first however quoting Bp. Pearson's words on its Conciliar authority. (Pearson, Exp. of the Creed, Art. III., n.37). "It is plain that the Council of Ephesus which condemned Nestorius confirmed this title Theotokos; I say confirmed it; for it is evident that it was before used in the Church, by the tumult which arose at the first denial of it by Anastasius [Nestorius's presbyter]; and so confirmed it as received before, because they approved the Epistles of St. Cyril, who proved it by the usage of those Fathers which preceded him."
(1) History of Word Theotokos.
It has not been unfrequently assumed that the word Theotocos was coined to express the peculiar view of the Incarnation held by St. Cyril. Such however, is an entire mistake. It was an old term of Catholic Theology, and the very word was used by bishop Alexander in a letter from the synod held at Alexandria in a.d.320,  to condemn the Arian heresy (more than a hundred years before the meeting of the Council of Ephesus); "After this, we receive the doctrine of the resurrection from the dead, of which Jesus Christ our Lord became the first-fruits; who bore a body in truth, not in semblance, which he derived from Mary the Mother of God (ek tes Theotokou Marias)."  The same word had been used by many church writers among whom may be mentioned St. Athanasius, who says, "As the flesh was born of Mary, the Mother of God, so we say that he, the Word, was himself born of Mary" (Orat. c. Arian., iij., 14, 29, 33; also iv., 32). See also Eusebius (Vit. Const., iij., 43); St. Cyril of Jerusalem (Cat., x., 9); and especially Origen, who (says Bp. Pearson) "did not only use, but expound at large the meaning of that title Theotokos in his first tome on the Epistle to the Romans, as Socrates and Liberatus testify."  (Cf. Origen in Deut. xxii., 23; vol. ij., p.391. A; in Luc. apud Galland, Bib. Patr., vol. xiv., append., p.87, D). A list is given by Dr. Routh, in his Reliquiæ Sacræ. Vol. ij., p.215 (1st Ed.), 332 (2d Ed.).
In fact Theodore of Mopsuestia was the first to object to it, so far as we know, writing as follows: "Mary bare Jesus, not the Word, for the Word was and remained omnipresent, although from the beginning he dwelt in Jesus in a peculiar manner. Thus Mary is properly the Mother of Christ (Christotocos) but not the mother of God (Theotocos). Only figuratively, per anaphoram, can she be called Theotocos also, because God was in Christ in a remarkable manner. Properly she bare a man, in whom the union with the Word was begun, but was still so little completed, that he was not yet called the Son of God." And in another place he says: "It is madness to say that God is born of the Virgin....Not God, but the temple in which God dwelt, is born of Mary."  How far Theodore had departed from the teaching of the Apostolic days may be seen by the following quotations from St. Ignatius. "There is one only physician, of flesh and spirit, generate and ingenerate, God in man, true Life in death, Son of Mary and of God, first passible and then impassible, Jesus Christ our Lord."  Further on in the same epistle he says: "For our God, Jesus the Christ, was borne in the womb by Mary etc."  With the first of these passages Bp. Lightfoot very aptly compares the following from Melito. "Since he was incorporeal, he fashioned a body for himself of our likeness...he was carried by Mary and clothed by his Father, he trod the earth and he filled the heavens." 
Theodore was forced by the exigencies of his position to deny the doctrine of the communicatio idiomatum which had already at that early date come to be well understood, at least so far as practice is concerned.
(Hefele, Hist. of the Councils, Vol. iii., p.8.)
This doctrine, as is well known is predicating the same properties of the two natures in Christ, not in abstracto (Godhead and manhood), but in concreto (God and man). Christ himself had declared in St. John iii., 16: "God...gave his only begotten Son" (namely, to death), and similarly St. Peter declared (Acts iii., 15): "ye...killed the Prince of Life," when in fact the being given up and being killed is a property (idioma = predicate) of man, not of God (the only begotten, the Prince of Life). In the same way Clement of Rome, for example, spoke of "the sufferings of God" (pathemata Theou) (1 Ad Cor.2), Ignatius of Antioch (Ad Ephes., c.1, and Ad Rom., 6) of an haima and pathos Theou, Tatian of a Theos peponthos (Ad Græcos, c.13); Barnabas teaches (c.7) that "the Son of God could not suffer except on our behalf...and on our behalf he has brought the vessel of his Spirit as a sacrifice." Similarly Irenæus (iii., 16, 6) says, "The Only-begotten impassible Word (unigenitus impassibilis) has become passible" (passibilis); and Athanasius, estauromenon einai Theon (Ep. ad Epictet., n.10, t. j., p.726. ed. Patav.)
It is, however, to be remarked that the properties of the one nature were never transferred to the other nature in itself, but always to the Person who is at the same time both man and God. Human attributes were not ascribed to the Godhead, but to God, and vice versâ.
For a full treatment of the figure of speech called the communicatio idiomatum the reader is referred to the great works on Theology where it will be found set forth at large, with its restrictions specified and with examples of its use. A brief but interesting note on it will be found in St. John Damascene's famous treatise De Fide Orthodoxa, Book III., iij. (Migne's Pat. Græc., col.994).
(2) Meaning of the Word Theotokos.
We pass now to the meaning of the word, having sufficiently traced the history of its use. Bishop Pearson says: "This name was first in use in the Greek Church, who, delighting in the happy compositions of that language, called the blessed Virgin Theotocos. From whence the Latins in imitation styled her Virginem Deiparam et Deigenitricem."  In the passage to which the words just quoted are a portion of a footnote, he says: "Wherefore from these three, a true conception, nutrition, and parturition, we must acknowledge that the blessed Virgin was truly and properly the Mother of our Saviour. And so is she frequently styled the Mother of Jesus in the language of the Evangelists, and by Elizabeth particularly the Mother of her Lord,' as also by the general consent of the Church (because he which was so born of her was God,) the Deipara; which being a compound title begun in the Greek Church, was resolved into its parts by the Latins and so the Virgin was plainly named the Mother of God."
Pearson is mistaken in supposing that the resolution of the compound Theotocos into meter tou Theou was unknown to the early Greek writers. Dionysius expressly calls Mary he meter tou Theou mou (Contr. Paul. Samos., Quæst. viij.); and among the Latins Mater Dei and Dei Genetrix were (as Pearson himself confesses in note 37) used before the time of St. Leo I. It is not an open question whether Mater Dei, Dei Genetrix, Deipara, meter tou Theou are proper equivalents for Theotokos. This point has been settled by the unvarying use of the whole Church of God throughout all the ages from that day to this, but there is, or at least some persons have thought that there was, some question as to how Theotocos should be translated into English.
Throughout this volume I have translated it "Mother of God," and I propose giving my reasons for considering this the only accurate translation of the word, both from a lexico-graphical and from a theological point of view.
(a) It is evident that the word is a composite formed of Theos = God, and tiktein = to be the mother of a child. Now I have translated the verbal part "to be the mother of a child" because "to bear" in English does not necessarily carry the full meaning of the Greek word, which (as Bp. Pearson has well remarked in the passage cited above) includes "conception, nutrition, and parturition." It has been suggested that "God-bearer" is an exact translation. To this I object, that in the first place it is not English; and in the second that it would be an equally and, to my mind, more accurate translation of Theophoros than of Theotokos.
Another suggestion is that it be rendered "the bringer forth of God." Again I object that, from a rhetorical standpoint, the expression is very open to criticism; and from a lexicographical point of view it is entirely inadequate, for while indeed the parturition does necessarily involve in the course of nature the previous conception and nutrition, it certainly does not express it.
Now the word Mother does necessarily express all three of these when used in relation to her child. The reader will remember that the question I am discussing is not whether Mary can properly be called the Mother of God; this Nestorius denied and many in ancient and modern times have been found to agree with him. The question I am considering is what the Greek word Theotocos means in English. I do not think anyone would hesitate to translate Nestorius's Christotocos by "Mother of Christ" and surely the expressions are identical from a lexicographical point of view.
Liddell and Scott in their Lexicon insert the word theotokos as an adjective and translate "bearing God" and add: "especially he Theotokos, Mother of God, of the Virgin, Eccl."
(b) It only remains to consider whether there is from a theological point of view any objection to the translation, "Mother of God." It is true that some persons have thought that such a rendering implied that the Godhead has its origin in Mary, but this was the very objection which Nestorius and his followers urged against the word Theotocos, and this being the case, it constitutes a strong argument in favour of the accuracy of the rendering. Of course the answer to the objection in each case is the same, it is not of the Godhead that Mary is the Mother, but of the Incarnate Son, who is God. "Mother" expresses exactly the relation to the incarnate Son which St. Cyril, the Council of Ephesus, and all succeeding, not to say also preceding, ages of Catholics, rightly or wrongly, ascribe to Mary. All that every child derives from its Mother that God the Son derived from Mary, and this without the co-operation of any man, but by the direct operation of the Holy Ghost, so that in a fuller, truer, and more perfect sense, Mary is the Mother of God the Son in his incarnation, than any other earthly mother is of her son.
I therefore consider it certain that no scholar who can and will divest himself of theological bias, can doubt that "Mother of God" is the most accurate translation of the term Theotocos.
 Cf. Bp. Lightfoot's criticism on Gibbon as an historian, The Apostolic Fathers, Vol. I., p. 46 n. Macaulay's History of England will of course instantly present itself to the reader as a sample of the brilliant variety of histories referred to in the text.  The date is not certain, it may have been a year or so different.  Theod., Hist. Eccl., I., 4.  Pearson, An Expos. of the Creed, Art. III., n. 36.  I take this passage as cited by Hefele, Hist. Counc., Vol. III., 9,  Ignat., Ad. Eph., vii.  Ibid. xviij.  Melito, Fragm. 14 (ed. Otto); cit. Lightfoot, Apost. Fath., II., 1, p. 48, n.  Pearson, An Expos. of the Creed, Art. III., n. 36.
 The date is not certain, it may have been a year or so different.
 Theod., Hist. Eccl., I., 4.
 Pearson, An Expos. of the Creed, Art. III., n. 36.
 I take this passage as cited by Hefele, Hist. Counc., Vol. III., 9,
 Ignat., Ad. Eph., vii.
 Ibid. xviij.
 Melito, Fragm. 14 (ed. Otto); cit. Lightfoot, Apost. Fath., II., 1, p. 48, n.
 Pearson, An Expos. of the Creed, Art. III., n. 36.