From antiquity, Mary has been called "Theotokos", or "God-Bearer" (Mother of God). The word in Greek is "Theotokos". The term was used as part of the popular piety of the early first millennium Church. It is used throughout the Eastern Church's Liturgy, both Orthodox and Catholic. It lies at the heart of the Latin Rite's deep Marian piety and devotion. This title was a response to early threats to orthodoxy, the preservation of authentic Christian teaching. A pronouncement of an early Church Council, the Council of Ephesus in 431 A.D., insisted "If anyone does not confess that God is truly Emmanuel, and that on this account the Holy Virgin is the "Theotokos" (for according to the flesh she gave birth to the Word of God become flesh by birth) let him be anathema." (The Council of Ephesus, 431 AD)
The Council's insistence on the use of the title reflected an effort to preserve the teaching of the Church that Jesus was both Divine and human, that the two natures were united in His One Person. Not only was that teaching under an assault then, it is under an assault now, and failing to "get it right" has extraordinary implications. The reason that the early Church Council pronounced this doctrine was "Christological", meaning that it had to do with Jesus Christ. One of the threats was from an interpretation of the teachings of a Bishop of Constantinople named Nestorius. Some of his followers insisted on calling Mary only the "Mother of 'the Christ'". The Council insisted on the use of the title (in the Greek) "Theotokos," ("Mother of God" or "God-bearer") to reaffirm the central truth of what occurred in the Incarnation of Jesus Christ.
Rejection of the truth revealed in this beautiful title of Mary has led to a diminution in the understanding and role of Mary, impeding some Christians from grasping a deeper truth concerning the meaning of Mary's life - her Fiat, her "Yes" to God's Will. It is a privation, leading to a reduced understanding of the call to every Christian to live our lives for God as Mary did. It has undermined our mission to bring the world to the new world, recreated in her Son, the Church which is His Body on earth and a seed of the Kingdom which is to come. The Church, of which we are members through baptism, continues His redemptive mission until he returns.
When we fail to receive the gift of Mary as Mother we can also miss the call of every Christian to bear Jesus for the world as she did. It is time to re-examine the deeper implications of the treasure that is found in the life example and message of the little Virgin of Nazareth. This wonderful title, Mary, the Mother of God, "Theotokos", reveals a profound truth not only about Mary, but about each one of us. We are now invited into the very relationship that she had with her Son. We can become "God-bearers" and bring Him to all those whom we encounter in our few short days under the sun.
(Additional commentary by St. Mark)
To deny the Divine Maternity of the Blessed Virgin is the heresy of Nestorianism. Nestorianism divides Christ into two Persons. This heresy was condemned at the Council of Ephesus in 431, and the same Council infallibly defined that Christ is one Person with two Natures (the Hypostatic Union) and since both Natures came through the Holy Virgin, though His Divinity preceded Her, She is truly Mother of God, Jesus Christ. Thus, sadly, when Protestants deny this venerable Title of Our Lady, they are really professing Nestorianism, whether knowingly or unknowingly.
It must also be understood that the title of Mother of God is the first of all titles for the Virgin. Her preeminence in salvation is due to Her being Mother of God, and because She consented with full heart and mind to become that very Tabernacle of the Living Word made flesh. Thus, any honour, privilege or title ascribed to Her is due to this one great title: Mother of God.
Also, by the very fact that She is Mother of God, and Our Lord is called our "Brother" because He stooped down to taken upon Himself our humanity, She thus becomes Mother to all Christ's faithful brethren, which is the Catholic Church, and thus She becomes in very deed our Mother. This Maternity is executed by Providence upon all the Church's children, and She has become the very mantle to which we fly in times of danger.
Without grasping this fundamental dogma concerning Our Blessed Mother, no man can truly understand the mystery of the Christian Religion, and thus cannot be set free by the Truth, which Christ Himself promised. Without understanding this great mystery of Our Lady, one cannot appreciate the immensity of Our Lord's loving condensention to become Son of Man. It is truly a remarkable, nay, sublime thought, that God should take unto Himself a Mother, like common man, and being subject to Her by the very Law He commanded to Moses, He deigned to show us the right observance of the Law, which He fulfilled and brought to perfection with the New Convenant which is faithfully executed by the Holy Catholic Church.
Since a mother gives birth to the entire person, both the flesh and spirit, even though she did not create the spirit since that comes from God, she is truly called "mother", as the Decalogue attests. The entire person, both flesh and spirit, is placed under her guidance and maternal love, and she takes it upon herself to tend to both: by feeding the flesh and caring for it; by teaching the True Religion to the spirit, so that it can have a hope of eternal life in the Kingdom. Thus, even though Our Lady did not create or precede the Divinity of Jesus Christ, nevertheless, She gave birth to the Person of Christ, constituting both the Divine and Human, so that She is truly called Mother of God.