In Galatians 4:4, Paul writes,
"But when the time had fully come, God sent his Son, born of a woman, born under the law..."
Paul does not speak of just any time, but God's time. He does not state that Jesus was born to Mary at a random time, a kind of "dumb-luck" moment as an accident of history, but rather at a time of God's own choosing, a time when human history, and more particularly, the history of Israel had reached its zenith, a certain divine-human moment, when God would begin his great act of salvation. The birth of Jesus began the great christological moment that would bring the salvation of the world. Advent is a reminder to us that God acts on our behalf in his own time. We have not been abandoned and left to our own devices.
In the midst of a world seemingly out of control, God has made it clear that, when all is said and done, God will have his way and the entire world will see the salvation made possible in Jesus. God has seen to this deliverance by raising his Son from the dead. Jesus was born, crucified, and raised in the fullness of God's time. We live in the time of God's fullness as well, and God will use us if we are willing.