The doctrine of Mary as the Theotokos (Latin Dei Genetrix, English Mother of God or Godbearer) has come up in the comments.
Can one be an orthodox Christian and deny that Mary is the Mother of God? Ultimately, Mary's title as Mother of God is essential to orthodox Christology. If Christ is God and Mary is His mother, then she indeed the Mother of God. Her child is divine.
Even St Elizabeth understood this reality over 400 years before the Council of Ephesus when she said to her kinswoman Mary:
Luke 1:43 And why is this granted me, that the mother of my Lord should come to me?
Mary is the mother of the Lord. Failure to affirm this title is a failture to affirm the teaching of Sacred Scripture and thus the teaching of the Holy Spirit.