Summary: Paul's theology of the incarnation is explored.
“When the fullness of time had come, God sent forth his Son, born of woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons.” 
Irreligious individuals occasionally and foolishly charge that there never was a birth of the Christ. Such individuals appear united in denying the miraculous events that are outlined in the Word of God. Such thoughtless individuals may not necessarily deny that there actually lived during that particular era a man known as Jesus of Nazareth, but they assume that Luke and Matthew invented and/or created Christian myths in an attempt to enhance His stature. Some of these scoffers are willing to concede that there may have been a teacher of some local repute who gave rise to these legends, but they doubt that he was any greater than any other religious leader.
Perhaps the strongest argument unbelievers present in support of their infidelity is the assertion that no other writers of Scripture appear to make mention of the miraculous birth. These infidels display their ignorance of Holy Writ through exposing their folly. Multiplied texts speak of the birth of the Son of God. During this holy season I have already presented the testimony of prominent witnesses, including Isaiah, the prophet of royal birth, and John, the beloved disciple. Today I appeal to the Apostle to the Gentiles. The Apostle was certainly aware of the birth of Jesus who is the Christ. Moreover, he was aware of the significance of that birth as is evident from even a casual perusal of his letters. Focus on one of his earliest letters as we learn of Christmas according to Paul.
CHRIST THE SON OF GOD WAS BORN AT A DEFINITE TIME. “When the fullness of time had come…” These glorious words show that the entry point of the Christian message is at the same time the turning point of history. Even our system of dating confesses this truth. It is now 2013 AD—anno Domini, THE YEAR OF OUR LORD. Apart from the words which are penned here, life offers no future, no hope. We are left without hope and without God in the world if this is not 2013 THE YEAR OF OUR LORD. But God has intervened in a way which brings effective and complete salvation—hope to the hopeless and help to the helpless. That is the Christmas message.
What do you suppose is meant by the Apostle’s phrase, “When the fullness of time had come?” The Greek word translated fullness of time means exactly that. The thought conveyed is fulfilment, especially as related to time. Even a casual acquaintance with the ancient world reveals something of the significance of the Spirit's Word through the Apostle. Christ was born during the era identified as enjoying Pax Romana. This is that period of time when the rule of one government insured world-wide peace. Peace imposed by conquering armies had extended over most of the civilised earth making travel and commerce possible in a way previously unrealised. Great roads linked the empire of the Caesars, ensuring free and uninterrupted communications with the furthest outposts of civilisation and providing rapid access to all corners of the empire.
An even greater factor unifying the diverse regions of the empire was the all pervasive language and culture of the Greeks. Greek was the lingua franca of the empire, being readily understood and read throughout the whole Mediterranean world. Not only linguistically, but also culturally, the ancient world was dominated by the prior educational advances of the Greeks. Therefore, politically, culturally and linguistically the world was unified—conditions which ensured a rapid spread of the Good News declaring the salvation of Christ the Lord.
Religiously, that ancient world was sunk in a moral abyss so deep that even pagans cried out against it. Spiritual hunger was everywhere evident as witnessed by the prevalent appeal within every social stratum for spiritual insight from religions introduced from the east. Those religions in apparent ascendancy within the empire were exactly those ancient religions which had once been confronted by the prophets of God when the Hebrew peoples first entered the Promised Land. The philosophers contributed in a negative sort of way, casting doubt on the old pagan systems of religion as they looked for some sort of unifying power behind all the polytheistic systems which had previously prevailed. The Jews themselves had made preparation for the coming of the Christ through the preaching of monotheism in some one hundred fifty synagogues located throughout the empire, and by their anticipation of a Messiah who would right the world's wrongs. That human longing for communion with God, that spiritual restlessness which pervaded all mankind insured a receptive audience once Christ was come. These religious longings insured that for the entire world, the time had indeed fully come.