Gregory the Great
Catena Aurea · Lukasevangelium 1
Then said Mary unto the angel, How shall this be, seeing I know not a man? And the angel answered and said unto her, The Holy Ghost 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.
(18 Moral. c. 20. super Job 27:21.) By the term overshadowing, both natures of the Incarnate God are signified. For shadow is formed by light and matter. But the Lord by His Divine nature is light. Because then immaterial light was to be embodied in the Virgin’s womb, it is well said unto her, The power of the Highest shall overshadow thee, that is, the human body in thee shall receive an immaterial light of divinity. For this is said to Mary for the heavenly refreshing of her soul.