Christ came into the world in "the fullness of time" (Gal 4:4). His First Coming was foreseen from all eternity and His Nativity was prepared from the beginning of the history of our salvation. The manner of His birth is significant. St. Leo I, "the Great" (+461; whose Christmas sermons deserve entries apart!) wrote in a letter:
His God in that "all things were made through Him and nothing was made without Him." (John 1:3) He is human in that He was "made from woman, made under the law". The nativity of His flesh shows His human nature. The virgin birth is an indicator of His divine nature. [ep. to Flavian 4]
Commenting on Luke 2:5, Gaius Marius Victorinus (+IV c.), a teacher of St. Jerome, wrote:
As there is a fullness in things, so there is in time. For each thing has its fullness in a full and copious perfection that abounds in everything. Christ is the fullness of things. The fullness of times is the consummation of freedom. So that His fullness may be whole and perfect Christ collects His members who are scattered, and in this way His fullness is achieved. So in the same way the fullness of times was achieved when all had become ripe for faith and sins had increased to the utmost, so that a remedy was necessarily sought in the judgment of all things. Hence Christ came when the fullness of time was completed. [Epistle to the Galatians 2.4.3-4]
We are made in Gods image and likeness and Christ came into the world to reveal man for fully to him (GS 22). We are radically in unity with Christ and, through Him, with each other. In this light, read the words of 3rd century Alexandria writer Origen (+254):
So, when He died, we died with Him, and when He rose, we rose with Him. Likewise, we were also circumcised along with Him. After His circumcision, we were cleansed by a solemn purification. Hence we have no need at all for a circumcision of the flesh. you should know that He was circumcised for our sake. Listen to Pauls clear proclamation. He says, "For in Him the whole fullness of deity dwells bodily, and you have come to fullness of life in Him, who is the head of all rule and authority. In Him also you were circumcised with a circumcision made without hands, by putting off the body of the flesh in the circumcison of Christ. And you were buried with Him through faith in the working of God, who raised Him from the dead." (Col 2:9-12) Therefore His death, His resurrection and His circumsion took place for our sake. [Homilies on the Gospel of Luke 14.1]
We discern in the Gospels an interesting pattern. The Second Person empties Himself of glory and becomes incarnate of the Virgin Mary. The eternal Word becomes a speechless child. He is lain upon the wood of the crib. He is pierced with metal and He sheds His Blood for our sake. The Incarnate Word Jesus Christ empties Himself of glory and enters His Passion. He stands mute before Pilate and the soliders. He is lain upon the wood of the Cross. He is pieced with metal and sheds His Blood for our sake. In each case He is bound to the Temple, first in His Presentation, finally when the lambs (which foreshadow Him) are being slaughtered in the Temple. All of this is for our sake.