The answer
It is a heresy to not refer to St. Mary as the Mother of God, Theotokos (Birth Giver of God). In deductive logic, we say if A=B, and B=C, therefore A=C. If St. Mary=Mother of Christ, and Christ=God, therefore, St. Mary=Mother of God.The Nestorian Heresy was debated and resolved in the Council of Ephesus in 431 AD, under the leadership of St. Cyril the Pillar of Faith. As a result, the Introduction to the Orthodox (Nicene) Creed was added.
Reference:
http://lacopts.org/articles/saint-cyril-of-alexandria-and-the-council-of-ephesus/
Answered by His Eminence Metropolitan Youssef (opens in a new tab)