The angel said to Mary, "The Holy Spirit will come upon you and the power of the Most High will overshadow you.  Therefore the child to be born will be called holy, the Son of God."  Mary said in reply, "Behold I am the handmaid of the Lord.  May it be done to me --  "fiat mihi" -- according to your word."

For ages after, this moment has been enshrined in Christian creeds very simply:  "Jesus was conceived by the Holy Spirit and born of the Virgin Mary."

Why this moment, this Incarnation of God?  Athanasius quoting Ireneus gives the classic Christian belief:  "God became human, so humans could become God."  How is this possible?

Jesus is conceived by the Holy Spirit, and each of us, in baptism, is born again of water and the Spirit -- the very same Spirit of Jesus!  For us the promise and challenge of the Annunciation-Incarnation mystery is our personal "fiat" to this Spirit, inviting the
Spirit to be the guiding wisdom and energizing power of our lives -- just as it was for Jesus.  Then we become in fact -- not just in name -- the Body of Christ for our communities, incarnating the presence of Jesus for our world. 

And Mary, the mother of Jesus, becomes in fact -- not just in name -- our mother as we allow the Spirit of her son Jesus to abide in our hearts and bring us to full stature in her son. 

Then is the true promise of the Incarnation of the Lord realized:  "God became human, so humans could become God!"