Today in 431 AD, the Council of Ephesus, led by St Cyril of Alexandria, confirmed the Catholic Creed of Nicea against the heresy of Nestorius and decreed that Jesus Christ is God and Man in one person. Nestorius held that Jesus is a Divine Person (the Second Person of the Trinity) but that he is also a Human Person (Jesus Christ). The council however held that He is not two persons but that Jesus Christ is God and Man in the same person. Therefore Mary, who is the Mother of Jesus Christ, is therefore the Mother of God.
The term given was Theotokos (in Greek) or Dei Para (in Latin) and was a celebration not only of the doctrinal decision about Jesus, but also a celebration about the recognition of the unique dignity of Mary. Indeed, when the council was over and the declaration promulgated, the bishops were lifted on the crowds shoulders and paraded through the streets of Ephesus.
One cannot over-estimate the importance of this decision on the early Church, on Western & Eastern Civilization and on European culture from then onwards. Both the Eastern Catholic Church (even those churches that subsist after the Great Schism in the 11th Century) and the Western Catholic Church hold this is a dogma. It is only when one glimpses the enormity of this decision that one can begin to grasp the Catholic view and veneration of Mary, and the birth of the subject of Mariology.