Since the Council of Ephesus, A.D. 431, the Church has unhesitatingly proclaimed her faith in the divine maternity of our Lady. There had been much discussion on the point at first, for it was only gradually that the Christian people began to consider as to how our Lord could be God and Man at the same time. Heretics had arisen to assert that He was either one or the other, so that the Church was forced to define precisely what was the true teaching of our Lord. Then when He had been declared to be God and Man one single Person subsisting in two natures, human and divine the further point was immediately raised as to whether it was to be judged correct to speak of His Mother as the Mother of God. Only by degrees did this come up for discussion. It was clear, indeed, that our Lady could not be the mother of the Godhead: God, as God, could not have been born of her, since He was of eternity, and she but a creature of time. As God He could not be her son, but as Man He was obviously her child. Now did this constitute her His mother in such a way that she could rightly be spoken of as the Mother of God? It was a question, in other words, of precise terminology, for in all these matters it is common knowledge how careful the Church has always been to be perfectly accurate. The position, then, was to discover the tradition of Christian people, and it was in Ephesus that the Bishops were gathered together who were to announce what had been the constant teaching of the Church.
It was a coincidence that at Ephesus this decision had to be taken, for it was at Ephesus that our Lady had lived after the Apostles had been dispersed. When she had been confided to the care of Saint John by our Lord on the Cross, this had been interpreted to mean that the young Apostle was to have charge of her while she yet remained separate from her Son. When, then, John came himself to settle at Ephesus in the position of bishop or chief of the Christian community there assembled, it was only natural that she should accompany him and live with him. This, at least, is the ordinary account that is given us of the matter, and still at Ephesus is shown " the Virgin's house." The decision of the bishops can be easily guessed. They declared her to be rightly acclaimed the Mother of God, and since that date the title has been unfailingly assigned to her in the official liturgy of the Church. The basis of this claim is the doctrine of the Incarnation. Our Lord came to redeem us, to atone for the sins of the human race. This He could not do unless He was at once God and man, man that he might suffer, God that His sufferings might have infinite avail. Moreover, before He became man He was already a person, being indeed the Second Person of the Holy Trinity: consequently there was no reason why He should take to Himself a new personality. The Divine Personality alone could enable Him to fulfil all justice, consequently when He was born, He was born as man, but it was a Divine Person who was born as man. Man by nature, He still retained His unique Divine Personality. Our Lady, therefore, is the mother of Him who is God.
The importance of this decision can hardly be overestimated, since it is absolutely on it alone that rests the whole reason for our especial devotion to our Lady, above the devotion that we show to all the saints. Her whole position in the Catholic world depends upon the acceptance of this truth: we believe, that is to say, that if she was chosen to be His mother, she must have been fitted for her work. Whoever is singled out by God for a special place on earth is no doubt prepared by every necessary grace for that place. If, then, out of all humanity He fitted our Lady to be His mother, it is obvious that she must have been made ready by every possible grace for her close relationship to Him. To no other was it granted to have such intimate acquaintance with Him. For months He abode within her womb; for years He was tended by her alone, left to her sole care and control: surely, then, we have a right to assume that she must have been fitted above all others to sustain so absolute a trust. He would not have permitted one so closely related to His own body to see corruption, for of her very flesh was that body formed. From the moment, then, that she accepted her high destiny from the angel's voice, there shone in her eyes the light of motherhood. Without human intervention or concurrence, by her sole union with the Deity, was born her Son. Surely, then, with truth did the angel hail her as full of grace. Because she is the Mother of God, we draw to her with confidence, assured that from her own humanity will spring her wish to help us, and from her close union with God her power to do so.
- text taken from Meditations for Layfolk by Father Bede Jarrett, O.P.