A Christmas answer to a friend:
Hi ****, there’s a difference between simply “became fully God” and “became fully God and fully man”. The genius of the incarnation is still a far off thing to most contemporary christians, not for lack of trying to understand.
In this, While the eternal son of God was and is always fully God, the scriptures teach “And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.”
And so while we always understand that the Son of God, is God, we might not always understand that Jesus became fully God and fully man at a specific point in history.
Prior to the incarnation Jesus was not fully man, nor in fact any part man at all. His humanity had not yet been created any more than it had been born of the virgin or liable to die. There was a change that was needful prior to our justification in that the Son of God took upon him our humanity, becoming like us, in order to save us. His humanity, like ours, was neither eternal nor immortal. The capacity for mortality was a large part of the reason for his incarnation.
This has to do with Christmas in that this is the time of year the culture has chosen to remember and celebrate the incarnation of the Son of God – and it’s a good time to remember it as it truly is. Not as a godlike man-child in a manger or a god disguised as a babe in swaddling cloth but as fully God, and fully man, yet one person, so as to communicate the virtues of each, without confusion or mixture of their distinct properties.
Here the God-man came, able to die in our place and stead, but also risen by the power of an imperishable life, because death could not hold him.
“But God raised him from the dead, freeing him from the agony of death, because it was impossible for death to keep its hold on him.” Acts 2:24
And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth. John 1:14
Who, existing in the form of God, did not consider equality with God something to cling to, but emptied Himself, taking the form of a servant, being made in human likeness. Philippians 2:6
For in Him all the fullness of Deity dwells in bodily form. Collossians 2:9
“The Word perceived that corruption could not be got
rid of otherwise than through death; yet He Himself,
as the Word, being immortal and the Father’s Son,
was such as could not die. For this reason, therefore,
He assumed a body capable of death, in order that it,
through belonging to the Word Who is above all,
might become in dying a sufficient exchange for all,
and, itself remaining incorruptible through His
indwelling, might thereafter put an end to corruption
for all others as well, by the grace of the
resurrection.” On the Incarnation, Athanasius
“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 soul and body; 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 taught us, and the Creed of the holy Fathers has handed down to us.” The Creed of Chalcedon