There seems to be some misunderstanding in evangelical circles as to what happened when the Son of God became man at the birth of Jesus Christ. Often these misconceptions can be blamed on poor theology in some of our most cherished Christmas carols.
1) "He laid his glory by"
This line appears in carol after carol. But really? Certainly, Jesus came in appearance as a man. His glory was veiled as he came in the likeness of sinful flesh. But his glory as the Son shone as brightly as ever. What could it conceivably mean that Jesus as the Son of God "laid his glory by"? His glory is the full expression of his godhead. He could not abandon one iota of his glory without ceasing to be God. If he stopped being God at the incarnation, how could be be Immanuel, God with us? Besides, even as the incarnate Word, his glory was not exactly "laid by". Doesn't Scripture say that "the Word became flesh and we beheld his glory" (John 1:14)? If we sing carols that say, "He laid his glory by" let us think in terms of glory veiled, not abandoned.
2) "He left his throne and his kingly crown"
No he didn't. As the Son of God he continued to rule over creation and direct providence. He did not abdicate his role in the Lordship of the Trinity. On the other hand, Scripture tells us that he was born, "the king of the Jews" (Matthew 2:2). Jesus came to take a new, additional crown, rather than abandon one. Because of his enfleshment, holy life and sacrificial death, Jesus has been crowned King of kings and Lord of lords, "And being found in appearance as a man, He humbled Himself and became obedient to the point of death, even the death of the cross. Therefore God also has highly exalted Him and given Him the name which is above every name" [emphasis added] (Phil 2:8-9). Jesus now exercises the crown rights of the Redeemer as well as Creator. As the world's true Lord and King, he will rescue the creation from the disastrous effects of sin and bring the created order back into subjection to God.
3) "Jesus became a human person"
You won't find that in any carols (thankfully!), but I have sometimes heard it said by well-meaning Christians. But Jesus didn't become a human person. What happened at the incarnation was that the person of the Son took human nature. If we say that the Son (as second person of the Trinity) became a human person, then Jesus is a combination of two persons. In that case, he has two identities and two centres of self-consciousness. But the Jesus revealed in the Gospels is one person; the Son, with a human nature. He may have two levels of consciousness - as the divine Son he knows all things, in his humanity he only knows some things. But in both his divine and human natures, he was conscious of only one personal identity - that of God's well beloved Son.
That's enough bad Christmas theology for now. Here is some good Christmas theology,
Behold the great Creator makes
himself a house of clay,
a robe of virgin flesh he takes
which he will wear for ay.
Hark, hark, the wise eternal Word,
like a weak infant cries!
In form of servant is the Lord,
and God in cradle lies.
This wonder struck the world amazed,
it shook the starry frame;
squadrons of spirits stood and gazed,
then down in troops they came.
Glad shepherds ran to view this sight;
a choir of angels sings,
and eastern sages with delight
adore this King of kings.
Join then all hearts that are not stone,
and all our voices prove,
to celebrate this holy One,
the God of peace and love.
Thomas Pestel (ca. 1586-1660)