In this excerpt from Truths We Confess, R.C. Sproul describes the timing of the incarnation of Jesus as it is summarized in the Westminster Confession of Faith.
The Son of God . . . did, when the fullness of time was come, take upon him man’s nature, with all the essential properties, and common infirmities thereof, yet without sin; being conceived by the power of the Holy Ghost, in the womb of the virgin Mary, of her substance. (WCF 8.2)
The fullness of time is a concept taken from the New Testament (Gal. 4:4). It places redemption in the arena of ordinary history. God is working out salvation in and through history. The Old Testament reveals to us God’s progressive unfolding of His divine plan of salvation, beginning with Adam and Eve and continuing with Noah, Abraham, Isaac, Jacob, and Moses. There is a clear progression of redemptive history, and a developing clarity in what God reveals to His people. Christianity is inseparably bound up with history. It is married to time and space, not something that occurs merely in some spiritual sphere. That is why the New Testament makes such statements as this: “In those days a decree went out from Caesar Augustus that all the world should be registered. This was the first registration when Quirinius was governor of Syria” (Luke 2:1–2). There was a historical setting for the entrance of Christ into the world. The incarnation happened in real history. The Roman governor, Pontius Pilate, is mentioned in the Apostles’ Creed. Christianity is about real people, places, and events. God promises us that there is a future for the people of God in time and space.
In Galatians 4:4, the “fullness” of time is the plērōma of time. The English language falls short in translating that idea. If we took a glass and filled it to the brim with water, we would say that the glass is full. But plērōma suggests a glass under the faucet, with water cascading over the sides of the glass, full to the point of overflowing. This is what the Bible means when it refers to the “fullness” of time. History was overflowing with anticipation; all of history, from the creation onward, was converging in that moment in history when Jesus Christ would be born.