This passage has become the most meaningful to me in reference to the incarnation of Christ, the event we celebrate today. Mainly because it expresses so many great truths about not just the event of Christ's advent, but the purpose and sovereign will of God in action, and the impact of that event.
We see the perfection of the timing of God's foreordained plan of sending God the Son to earth. He did so "when the fullness of the time came", at the point in human and redemptive history that God had appointed from before the foundation of the world. I love that term "fullness", speaking of all that God had purposed to occur leading up to the event being brought to realization. The sovereign purpose and plan of God the Father being worked out over thousands of years of history, all in preparation for the entry of God the Son into human form, the Word made flesh. Not an afterthought, not a Plan B, but the decisive action of God at just the time that He had determined, and in just the circumstances that He had planned.
The circumstances being noted next - "born of a woman, born under the Law." God the Son, the Christ, was to be conceived by supernatural means, but born by natural means, although to a virgin. No descending from heaven in a flash of glory, nothing showy or noteworthy - except to those few humble ones He chose to announce the event to. A child born of a woman in the most unlikely of places. Again, God's foreordained plan in action. And don't miss the significance of the Son being born to Jewish parents, "under the Law." The Christ was born into the nation of Israel who were commanded to keep God's Law - and could not. And the Christ was born under the Law so as to be the only human ever to meet the just requirements of the Law perfectly - so that He would be the righteous One, without sin. As the perfectly prepared sacrifice for the sin of the elect, and as the righteousness-earner for them as well.
And this then brings us to the purpose of all this. The "so that" part of the passage. The purpose of Christ's advent? Redemption, of those under the Law. Referring not just to the Jews, who had received the law of God, but to all people everywhere, who are accountable to God's perfect standard expressed in the Law. Every one of us is accountable to the Law of God, and fall short of His glorious standard, and therefore are condemned by the Law, unable to bear its burden. Christ's perfect fulfillment of the Law earned righteousness for the elect, and His perfect sacrifice earned forgiveness of sin for the elect. Here's the purpose of the grand plan of redemption that was determined in eternity past, was initiated at the birth of Christ, accomplished at the Cross of Christ, and will be consummated at the second advent of Christ.
And note also the result expressed here. The outcome of redemption is that "we might receive the adoption as sons." The position of the one who trusts in the redeeming person and work of Jesus Christ is not just declared forgiven and righteous in God's sight, but also in deep and eternal relationship with Him as Father. Adopted into the family of God. Brothers and sisters with Christ Himself. And as Paul goes on to say, joint heirs with Him, reigning with Jesus as our brother-King for eternity, to the glory of God the Father. An incredible inheritance, all of grace, and all for His glory.
This Christmas day, don't miss the grand unfolding of God's eternal plan. Christ's birth is just one part of that plan, a key one but also one that must be kept in context if we are to celebrate it and Him rightly. Praise God for His superintending "the fullness of the time" in Christmas and in all things.