We are often told that the Council of Ephesus (A.D 431) "defined" St Mary as Theotokos. In fact, it did no such thing. The Council issued no dogmatic decrees or definitions at all. The fathers condemned Nestorius as an heretic but the issue at stake was thoroughly Christological. The heresy of Nestorius lie in his position that Christ was a mere man and his teaching that St Mary was, therefore, Anthropotokos (Mother of the Man) was treated as evidence of this. But most of the evidence given against Nestorius came from his sermons that said absolutely nothing about St Mary. As Professor Price notes in his essay Theotokos: Significance in Doctrine and Devotion, the closest the Council came to defining St Mary as Theotokos was in an epistle sent by St Cyril of Alexandria to John of Antioch in A.D 433, two years after the synod, even so:
"On the matter of how we think and speak concerning the Virgin the Theotokos and the manner in which the only-begotten Son of God became man, we must state briefly (not by way of addition, but in the form of giving an assurance) what we have held from the first...We acknowledge our Lord Jesus Christ, the only-begotten Son of God, perfect God and perfect Man made up of a rational soul and body, begotten from the Father before the ages in respect of the Godhead and the same on the last day for us and for our salvation from the Virgin Mary in respect of his manhood...By virtue of this understanding of the union which involves no merging, we acknowledge the holy Virgin to be Theotokos, because God the Word was enfleshed and became man and from the very conception united to himself the temple taken from her."
I take comfort in the fact that these early debates were fundamentally about Christ, not His Mother.