“The angel

Said to her.

‘The Holy Spirit

Will come upon you.

The power

Of the Most High

Will overshadow you.

Therefore,

The child to be born

Will be holy.

He will be called

The Son of God.’”

καὶ ἀποκριθεὶς ὁ ἄγγελος εἶπεν αὐτῇ Πνεῦμα Ἅγιον ἐπελεύσεται ἐπὶ σέ, καὶ δύναμις Ὑψίστου ἐπισκιάσει σοι· διὸ καὶ τὸ γεννώμενον ἅγιον κληθήσεται Υἱὸς Θεοῦ.

Luke then has the Angel Gabriel reveal the whole plan.  Just as in Matthew, chapter 1:20, the angel told Joseph that the child conceived in Mary was from the Holy Spirit.  Luke did the same here.  This Angel Gabriel answered Mary (καὶ ἀποκριθεὶς ὁ ἄγγελος εἶπεν αὐτῇ).  He said that the Holy Spirit would come over her (Πνεῦμα Ἅγιον ἐπελεύσεται ἐπὶ σέ).  The power of the Most High God would overshadow or envelop her (καὶ δύναμις Ὑψίστου ἐπισκιάσει σοι).  Her child would come forth or be born holy (διὸ καὶ τὸ γεννώμενον ἅγιον).  He would be called the Son of God (κληθήσεται Υἱὸς Θεοῦ).  This was a very theological statement uttered by the Angel Gabriel.  This child, Jesus would come about because of the Holy Spirit.  Elohim, the most high God, would overshadow her, the way that Yahweh had overshadowed the tent of dwelling with the covenant in Exodus, chapter 37:1-9.  This presence of God in Mary would make her pregnant through the power of the Holy Spirit.  The unique act of Jesus’ conception was a fully Trinitarian action involving the Father, the Son, and the Holy Spirit within the womb of Mary.  Obviously, the Son, her son, Jesus, would be born as a holy human person, clearly and rightfully called the Son of God.  This was a very developed theology of Luke, who always stressed the role of the Holy Spirit in his writings.