Council of Ephesus
The Council of Ephesus met in A.D. 431 to condemn the heresy of Nestorianism, a heresy that taught that there were two distinct persons in Jesus Christ, a human and a divine. Consequentially, it affirmed the right of Mary to be called "Mother of God". It affirmed the Twelve Anathemas of Cyril of Alexandria and exiled Nestorius to the Egyptian desert.
The Council re-affirmed that Christ was truly God made Man. It was not the case, as some pretended, that Jesus Christ was merely a Man in Whom God dwelt in a special way. Since Christ was truly God, Mary the Mother of Christ was truly the Mother of God. It also follows as true that God died for our sins i.e. God chose to suffer death in the flesh for our sake.
- The Council of Ephesus – 431 A.D. (papalencyclicals.net) includes letters regarding Nestorius
- The Canons of the Ecumenical Councils of the Eastern Orthodox Church: Council of Ephesus (431) (sites.google.com)
- 0431–431 –Concilium Ephesenum – Documenta Omnia. The Third Ecumenical Council. The Council of Ephesus (70 pages) pdf (documentacatholicaomnia.eu) 
- http://www.documentacatholicaomnia.eu/03d/0431-0431,_Concilium_Ephesenum,_Documenta_Omnia_[Schaff],_EN.pdf (must copy and pasted the entire pdf into your browser, not merely click on it)