While I believe Scripture speaks for itself on this topic, I think this summary of Nestorianism (Copied from this Web (
catholic.com/library/great_heresies.asp) shows how the One, Holy Catholic and Apostolic Church reacted to ideas of this sort:
Nestorianism (5th Century)
This heresy about the person of Christ was initiated by Nestorius, bishop of Constantinople, who denied Mary the title of Theotokos (Greek: “God-bearer” or, less literally, “Mother of God”). Nestorius claimed that she only bore Christ’s human nature in her womb, and proposed the alternative title Christotokos (“Christ-bearer” or “Mother of Christ”).
Orthodox Catholic theologians recognized that Nestorius’s theory would fracture Christ into two separate persons (one human and one divine, joined in a sort of loose unity), only one of whom was in her womb. The Church reacted in 431 with the Council of Ephesus, defining that Mary can be properly referred to as the Mother of God, not in the sense that she is older than God or the source of God, but in the sense that the person she carried in her womb was, in fact, God incarnate (“in the flesh”).
My two cents:
In order for Christ to bring salvation to humanity, he must be fully human. So, my question here is: From where did he get his Humanity? If not from his Earthly mother, than whom?
If not her, than you start messing with the “Fully human, Fully Divine” Doctrine, and if you mess with that, you start messing with the meaning of the sacrifice of the cross, which I’m sure most Protestants would rather not do.
God Bless!