On the True Theotokos

St. Gregory the Theologian ca. 329-389

If anyone does not believe that Holy Mary is the Mother of God, he is severed from the Godhead. (Epistle 101: To Cledonius)

St. Cyril of Alxandria ca. 376-444

If any one confess not that Emmanuel is in truth God and that the holy Virgin is therefore Mother of God, for she bare after the flesh the Word of God made Flesh, be he anathema. (Third Letter to Nestorius, Anathema 1)

St. Maximus the Confessor ca. 580-662

In the name of the Father, Son, and Holy Spirit, the co-essential and supersubstantial Trinity; of all the hosts of heaven; of the choir of holy apostles and prophets; of the innumerable array of martyrs; and of every righteous soul reposed in the faith: may he be anathema who fails to confess our all-hymned, all-holy, immaculate Lady, the most honorable of rational beings, as the true Mother of God, Who hath made heaven and earth, the sea and all that is therein, now and ever and unto ages of ages. (St. Dimitri Rostov, Life of St. Maximus)