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. And, behold, thy cousin Elisabeth, she hath also conceived a son in her old age: and this is the sixth month with her, who was called barren. For with God nothing shall be impossible. - Luke 1:34-37

Mary is not doubting Gabriel’s words. She is truly wondering how she could become pregnant since she is a pure virgin (Matthew 1:18). The term "know" is the modest way the Bible describes an intimate sexual relation (Genesis 4:1).

Gabriel does not see her as doubting so he offers no rebuke, but he does offer an explanation of how the Word will be made flesh (John 1:14). We should notice that Gabriel’s description is very detailed but sounds nothing like the heathen fables of mythology where their gods have adulterous relations with humans. There is nothing sexual described by Gabriel. As a matter of fact, this work is not ascribed to the Father or the Word/Son; The Holy Spirit is the one who miraculously formed the body from the seed of Mary in her womb. The same way the Spirit moved in creation and set life in motion, He will move deep within Mary and bring life.

When the body of Jesus was conceived, the Word was united with that body; and that body took on the name the Word already held for all eternity, "the Son of God." The human nature was united to the already existing Son of God; and therefore, it should instantly bear the same name. The baby is referred to as holy. That means that the Spirit sanctified it at the very start of conception so that it never has, nor ever will contain the taint of original sin.

Gabriel offers further proof of what he says by telling Mary that God already performed another miracle with Elisabeth, her cousin. Just as Abraham and Sarai learned, Mary can properly answer the question,

Is anything too hard for the LORD? - Genesis 18:14a