So the holy one to be born will be called the Son of God

Luke 1:35
The angel answered, “The Holy Spirit will come on you, and the power of the Most High will overshadow you. So the holy one to be born will be called the Son of God. 

Mary was chosen for the Son of God to be physically born inside her. Believers in the Son of God, are also chosen to have the Holy Spirit born inside us. Mary's response was a positive acceptance and obedience as she surrendered herself to God's will. So believers too are expected to surrender their wills to the Holy Spirit who lives in us and guides us in our lives. He who lives in us, does not control us, so we can still make mistakes, but to allow ourselves to follow Him would be walking according to God's will.