It is false, and I believe heretical, doctrine to say that God the Son became human!
Does it make any difference whether we say God the Son became man or that He took on the form of man? In the Incarnation God the Son, the Second Person of the Godhead, laid aside His Glory but not His Divinity. He took upon Himself the nature of man, both body and soul, but He retained all the characteristics of Deity. In the Incarnate Son there is one person but two natures. Jesus Christ was truly God and truly man. The Apostle Paul writes of the divine-human natures of Jesus Christ as follows:
John 1:14
14. And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
John Gill writing about John 1:14 states:
“The same word, of whom so many things are said in the preceding verses; and is no other than the Son of God, or second person in the Trinity; for neither the Father, nor the Holy Ghost, were made flesh, as is here said of the word, but the Son only: and "flesh" here signifies, not a part of the body, nor the whole body only, but the whole human nature, consisting of a true body, and a reasonable soul; and is so called, to denote the frailty of it, being encompassed with infirmities, though not sinful; and to show, that it was a real human nature, and not a phantom, or appearance, that he assumed:and when he is said to be "made" flesh, this was not done by the change of one nature into another, the divine into the human, or the word into a man; but by the assumption of the human nature, the word, taking it into personal union with himself; whereby the natures are not altered; Christ remained what he was, and became what he was not; nor are they confounded, and blended together, and so make a third nature; nor are they separated, and divided, so as to constitute two persons, a divine person, and an human person; but are so united as to be but one person; and this is such an union, as can never be dissolved, and is the foundation of the virtue and efficacy of all Christ's works and actions, as Mediator:”
Colossians 2:9,
9. For in him dwelleth all the fulness of the Godhead bodily.
Hebrews 2:16
16. For verily he took not on [him the nature of] angels; but he took on [him] the seed of Abraham.
Hebrews 10:5
5. Wherefore when he cometh into the world, he saith, Sacrifice and offering thou wouldest not, but a body hast thou prepared me:
Romans 8:3
3. For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh:
Philippians 2:7
7. But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men:
There is absolutely nothing in the above Scripture describing the incarnation which states or implies that God the Son became human. This issue was addressed at the Council of Chalcedon as follows:
Chalcedonian Creed (451 A.D.)
This creed was adopted at the Fourth Ecumenical Council, held at Chalcedon, located in what is now Turkey, in 451, as a response to certain heretical views concerning the nature of Christ. It established the orthodox view that Christ has two natures (human and divine) that are unified in one person.
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We, then, following the holy Fathers, all with one consent, teach men to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a reasonable [rational] soul and body; consubstantial [co-essential] with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood; one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son, and only begotten, God the Word, the Lord Jesus Christ; as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed of the holy Fathers has handed down to us.
I would call the attention of all those who believe that Mary is the mother of God that Chalcedon simply states: Mary, the Mother of God, according to the Manhood.. Chalcedon states that Mary was the mother of the Human Nature of Jesus Christ, not the mother of the divine nature, not the mother of God.