If that is true, then by definition, His human nature is not God, since God is eternal and unchanging.
Therefore Mary cannot be the Mother of God.
"And she spake out with a loud voice, and said, 'Blessed art thou among women, and blessed is the fruit of thy womb. And whence is this to me, that the mother of my Lord should come to me?'" [Luke 1:42-43]
Note later what Calvin will say about this verse...
And the witnesses of the Church:
"Though still a virgin she carried a child in her womb, and the handmaid and work of his wisdom became the Mother of God" (Ephraim the Syrian, "Songs of Praise" 1:20 [A.D. 351]).
"The Word begotten of the Father from on high, inexpressibly, inexplicably, incomprehensibly, and eternally, is he that is born in time here below of the Virgin Mary, the Mother of God" (Athanasius, "The Incarnation of the Word of God" 8 [A.D. 365]).
"If anyone does not agree that Holy Mary is the Mother of God, he is at odds with the Godhead. If anyone asserts that Christ passed through the Virgin as thought a channel, and was not shaped in her both divinely and humanly, divinely because without man and humanly because in accord with the law of gestation, he is likewise godless." [St. Gregory of Nazianz "Letter to Cledonius the Priest" 382 A D]
"When, therefore, they ask, `Is Mary mother of man or Mother of God?' we answer, `Both!' The one by the very nature of what was done and the other by relation. Mother of man because it was a man who was in the womb of Mary and who came forth from there, and the Mother of God because God was in the man who was born" (Theodore of Mopsuestia, "The Incarnation" 15 [A.D. 405]).
"[T]he Word himself, coming into the Blessed Virgin herself, assumed for himself his own temple from the substance of the Virgin and came forth from her a man in all that could be externally discerned, while interiorly he was true God. Therefore he kept his Mother a virgin even after her child-bearing" (Cyril of Alexandria, "Against Those Who Do Not Wish to Confess That the Holy Virgin is the Mother of God" 4 [A.D. 430]).
And even some other charecters you may recognize:
"It is a sweet and pious belief that the infusion of Mary's soul was effected without original sin; so that in the very infusion of her soul she was also purified from original sin and adorned with God's gifts, receiving a pure soul infused by God? thus from the first moment she began to live she was free from all sin." [Martin Luther; "Sermon On the Day of the Conception of the Mother of God", 1527]
"It cannot be denied that God in choosing and destining Mary to be the Mother of His Son, granted her the highest honor... Elizabeth calls Mary Mother of the Lord, because the unity of the person in the two natures of Christ was such that she could have said that the mortal man engendered in the womb of Mary was at the same time the eternal God." [John Calvin; quoted in "Corpus Reformatorum", v. 45, p. 348]
Even the first reformers hadn't totally tossed Christianity to the wayside yet.
Regards
The divine nature is eternal and unchanging, but the Second Person of the Trinity is not identical to the divine nature. The Second Person of the Trinity has *two* natures, joined in a hypostatic union. And the Second Person of the Trinity is God. Therefore, if you had lived in Nazareth 2000 years ago, and pointed at Jesus and said, "Is that God?", the correct answer would be "Yes, that is God." And if you reached out and touched His arm or His foot, you would be touching God. See my post #1676.
-A8