First of all heresy, like treason, is a made up concept.It only applies in the third person “their heresy”. It is a term thrown around by individuals that get angry when people do not agree with them and they strike out in a very non-Christian way. You must understand that since we do not accept the authority of your church the word heresy is irrelevant. Now back to your comment; no one is dividing Jesus. Jesus is not two individuals but has two natures; fully human and fully divine. I think you will agree that Mary was a created being. Her assent was necessary for she above all others chose to fully submit to the perfect will of God. Now to your point: Jesus is God (we both agree), Mary was his mother (again we agree). Here is where we disagree. A created being (Mary) cannot give birth to the God that created her therefore not the Mother of God per say. I realize the historical reason Mary was given this title but in an effort to solidify doctrine the title serves to divide.The title Mother of God is essential to justify other Marian doctrines.
I do want to stress that discussions like this make it seem like Protestants have something against Mary and that is not true. We honor her as the most perfect example of surrendering to the will of God and her wonderful example of motherhood to Jesus. We love her too but we have a much different understanding of her and her role in God’s perfect plan of salvation.