For it is evident that our Lord sprang out of Judah, of which tribe no one turned his attention to the altar (Hebrews 7:14), as the divine apostle said….
And bearing glad tidings to her, he said, Hail thou highly favoured one, the Lord is with thee (Luke 1:28).
And she was troubled at his word, and the angel said to her, Fear not, Mary, for thou hast found favour with God, and shalt bring forth a Son and shalt call His name Jesus (Luke 1:3-31).
For He shall save His people from their sins (Matt 1:21). Hence it comes that Jesus has the interpretation Saviour.
And when she asked in her perplexity, How can this be, seeing I know not a man? (Luke 1:34), the angel again answered her,
The Holy Spirit shall come upon thee, and the power of the Highest shall overshadow thee. Therefore also that holy thing which shall be born of thee shall be called the Son of God (Luke 1:35).
And she said to him, Behold the handmaid of the Lord: be it unto me according to Thy word (Luke 1:38).
So then, after the assent of the holy Virgin, the Holy Spirit descended on her, according to the word of the Lord which the angel spoke, purifying her (Luke 1:27-28), and granting her power to receive the divinity of the Word, and likewise power to bring forth.
And then was she overshadowed by the enhypostatic Wisdom and Power of the most high God, the Son of God Who is of like essence with the Father as of Divine seed.
And from her holy and most pure blood He formed flesh animated with the spirit of reason and thought, the first-fruits of our compound nature – not by procreation but by creation through the Holy Spirit.
[…] Being by nature perfect God, He naturally became likewise perfect Man. He did not change His nature nor make the dispensation an empty show.
He became, without confusion or change or division, one in hypostasis with the flesh, which was conceived of the holy Virgin, and animated with reason and thought, and had found existence in Him.
He did not change the nature of His divinity into the essence of flesh, nor the essence of flesh into the nature of His divinity, and did not make one compound nature out of His divine nature and the human nature He had assumed.
John Damascene (c.675-749): De Fide Orthodoxa 3,2 [slightly adapted].