Not true. I guess all the Apostles and the earliest Christians who NEVER referred to Mary as the "Mother of God" must be heretics, too? Can you produce even ONE reliable source that proves Mary was given the title or addressed that way by the Apostles or their direct disciples? The Apostle John even took care of her after the crucifixion and he never said a word about her being addressed this way nor did he direct others to in his epistles. If, as you claim, it is a matter of grave heresy to NOT call Mary the Mother of God, then they sure dropped the ball on that one! I guess next you'll be calling everyone a heretic who rejects Mary's "perpetual" virginity, her "immaculate conception" or her bodily assumption?
Please also note NO ONE DENIED that Mary is the one who gave birth to Jesus Christ who IS the Son of God/Almighty God incarnate (in the flesh). Refusing to address her in the way you do is hardly the same thing. You have your OPINION, that's all. You also avoided addressing your OWN logic problem when you have to use additional words to clarify your terms. Don't think that is forgotten.
You will note that I said “denied.” That is not at all the same as never having used the title.