Why do Orthodox call Mary the Mother of God?

This actually has to do with an ancient heresy called Nestorianism. In Nestorianism, Nestorius was trying to separate Jesus the man from the Logos, the God. So he kind of created this image of Jesus Christ that he was kind of a human at some points and God at some points, but really like two persons schizophrenically inside the incarnation. And what the church fathers were adamant about was no, Jesus Christ was both fully God and fully man. It was not like oil and water. It was not like two people. He was one hundred percent a man and one hundred percent God, and you could not divorce one from the other. And so Theotokos means the Mother of God. This does not mean that she gave birth to his divinity, because Jesus Christ, or the Logos, was from all of eternity, begotten of the Father, never created. But within her virginal womb existed the real God man Jesus Christ, and she gave birth to God in the flesh.