The phrase "Mother of God" (Theotokos) is christological and soteriological, not mariological. To deny Mary this phrase, is to dishonor Christ (denying his true deity), separating the person of the Logos from the person of Jesus (making two separate persons). This is the heresy of Nestorianism, condemned at the Council of Ephesus in 431. Protestants are crypto-nestorians. Read "On the Unity of Christ," by St. Cyril.