This should not even have to be discussed by the Church if today’s Christians were not ahistorical. This subject was decided long ago at the Council of Ephesus in the year 431.
We confess, then, our lord Jesus Christ, the only begotten Son of God perfect God and perfect man of a rational soul and a body, begotten before all ages from the Father in his godhead, the same in the last days, for us and for our salvation, born of Mary the virgin, according to his humanity, one and the same consubstantial with the Father in godhead and consubstantial with us in humanity, for a union of two natures took place. Therefore we confess one Christ, one Son, one Lord. According to this understanding of the unconfused union, we confess the holy virgin to be the mother of God because God the Word took flesh and became man and from his very conception united to himself the temple he took from her. As to the evangelical and apostolic expressions about the Lord, we know that theologians treat some in common as of one person and distinguish others as of two natures, and interpret the god-befitting ones in connexion with the godhead of Christ and the lowly ones with his humanity. [from http://www.piar.hu/councils/ecum03.htm#The%20judgment%20against%20Nestorius]
That should be quite sufficient to quiet any question as to whether it is appropriate to call Mary the Theotokos (Mother of God or God-Bearer). Of course for some, this council was some evil consortium of apostate bishops of the apostate Church. But we will consider if this decision was solely theirs or merely the crystallization of Early Christian thought.
St. Hippolytus of Rome writes in his A discourse by the most blessed Hippolytus, bishop and martyr, on the end of the world, and on Antichrist, and on the second coming of our lord Jesus Christ.
Thus, too, they preached of the advent of God in the flesh to the world, His advent by the spotless and God-bearing Mary in the way of birth and growth, and the manner of His life and conversation with men, and His manifestation by baptism, and the new birth that was to be to all men, and the regeneration by the laver…. [ANF 5: 232; sect. I]
Peter of Alexandria in speaking of his life in The Genuine Acts of Peter refers to Mary as both “Ever-Virgin” and “the Most Blessed Mother of God.”
[T]hey came to the church of the most blessed mother of God, and Ever-Virgin Mary, which, as we began to say, he had constructed in the western quarter, in a suburb, for a cemetery of the martyrs. [ANF 6: 267]
In speaking of Simeon, Methodius makes the following comment in his Oration Concerning Simeon and Anna .
While the old man was thus exultant, and rejoicing with exceeding great and holy joy, that which had before been spoken of in a figure by the prophet Isaiah, the holy mother of God now manifestly fulfilled. [ANF 6: 387; par. VII]
And later in the same oration
Hail to thee for ever, thou virgin mother of God, our unceasing joy, for unto thee do I again return. [ANf 6: 393; par. XIV]
St. Cyril of Jerusalem, in his Catechetical Lectures, tells us of the many witnesses concerning Christ.
Many, my beloved, are the true testimonies concerning Christ. The Father bears witness from heaven of His Son: the Holy Ghost bears witness, descending bodily in likeness of a dove: the Archangel Gabriel bears witness, bringing good tidings to Mary: the Virgin Mother of God bears witness: the blessed place of the manger bears witness. [NPNF 2-07: 61; lect. X; par. 19]
In To Januarius, Bishop of Caralis (Cagliari), Saint Gregory the Great (aka Gregory the Dialogist) writes concerning a domestic disturbance that has been brought to him. Note his very matter-of-fact use of the term Mother of Our God and Lord since it indicates the general usage that had developed.
The Jews who have come hither from your city have complained to us that Peter, who has been brought by the will of God from their superstition to the worship of Christian faith, having taken with him certain disorderly persons, on the day after his baptism, that is on the Lord’s day of the very Paschal festival, with grave scandal and without your consent, had taken possession of their synagogue in Caralis, and placed there the image of the mother of our God and Lord, the venerable cross, and the white vestment (birrum) with which he had been clothed when he rose from the font. [NPNF 2-13: 3; epistle VI]
St. Ambrose also uses the term in Concerning Virgins.
The first thing which kindles ardour in learning is the greatness of the teacher. What is greater than the Mother of God? What more glorious than she whom Glory Itself chose? What more chaste than she who bore a body without contact with another body? [NPNF 2-10: 374; bk. II; ch. II; par. 7]
John Cassian, of Conferences fame, powerfully defends the title Mother of God as a safeguard for the divinity of Christ in The Seven Books of John Cassian on the Incarnation of the Lord, Against Nestorius.
And so you say, O heretic, whoever you may be, who deny that God was born of the Virgin, that Mary the Mother of our Lord Jesus Christ ought not to be called Theotocos, i.e., Mother of God, but Christotocos, i.e., only the Mother of Christ, not of God. For no one, you say, brings forth what is anterior in time. And of this utterly foolish argument whereby you think that the birth of God can be understood by carnal minds, and fancy that the mystery of His Majesty can be accounted for by human reasoning, we will, if God permits, say something later on. In the meanwhile we will now prove by Divine testimonies that Christ is God, and that Mary is the Mother of God. [NPNF 2-11: 556; bk. II; ch. II]
And in the closing of the same chapter, he sums up his point.
And when the maiden understood not, he gave a reason for this great thing, saying: “Because the Holy Spirit shall come upon thee, and because the power of the Most High shall overshadow thee, therefore also that holy thing which shall be born shall be called the Son of God;” that is to say: That thou mayest not be ignorant of the provision for so great a work, and the mystery of this great secret, the majesty of God shall therefore come upon thee completely; because the Son of God shall be born of thee. What further doubt can there be about this? or what is there further to be said? He said that God would come upon her; that the Son of God would be born. Ask now, if you like, how the Son of God can help being God, or how she who brought forth God can fail to be Theotocos, i.e., the Mother of God? This alone ought to be enough for you; aye this ought to be amply sufficient for you. [NPNF 2-11: 557; bk. II; ch. II]
I can see no way in which I can make it any clearer than John Cassian already has!
