"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" (Romans 8:3).
The great truth of Christmas (meaning, originally, "Christ-sent") is that "God was manifest in the flesh" (I Timothy 3:16). The eternal Word "was made flesh, and dwelt among us" (John 1:14). So vital is this truth that "every spirit that confesseth not that Jesus Christ is come in the flesh is not of God, and this is that spirit of antichrist" (I John 4:3).
Jesus Christ is the God/Man--infinite God and perfect Man, perfectly joined in full union, and salvation is based on this truth. If Jesus Christ were not perfect Man, He could not die for the sins of man; if He were not God, He could not defeat death and save us from the penalty of sin.
He could not be born in sinful flesh, of course, like all the descendants of Adam, but only in the "likeness" of sinful flesh. From the moment of conception, He must be "holy, harmless, undefiled, separate from sinners" (Hebrews 7:26), and thus miraculously conceived in a virgin's womb.
In fact, that miraculous creation of His body in the womb of Mary was the actual moment when God became Man. It is even possible that the incarnation took place on about the very night that we now call Christmas, since it is probable that Jesus was actually born in the early fall, when shepherds were in the field with their sheep. It may even have been on Michaelmas ("Michael sent"), the fall holiday on September 29 honoring the angel who with the heavenly host announced the birth of Jesus on that night long ago. How appropriate it would be if "the light of the world" had indeed come into the world on or near that world's longest night just nine months before. HMM