Mary said to the angel, “How can this be, since I am a virgin?” The angel answered and said to her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; and for that reason the holy Child shall be called the Son of God.
When Gabriel told Zacharias that he and Elizabeth would become parents in their old age, Zacharias questioned the angel out of doubt. Mary's question to Gabriel was not prompted by doubt, but rather by her innocence. She was a young girl, a virgin and a devout follower of God. She had saved herself for her future husband. How then could she become pregnant and bear a child? Gabriel explained how she would conceive and why the immaculate conception was critical.The Savior must be born of a woman, so that He would be of the same nature as those whom He came to save. "But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law, so that He might redeem those who were under the Law, that we might receive the adoption as sons." (Galatians 4:4-5). And the Savior must be holy and without sin. "For we do not have a high priest who cannot sympathize with our weaknesses, but One who has been tempted in all things as we are, yet without sin." (Hebrews 4:15). Both of these requirements were met in Jesus -- fully man and yet fully God. “The Son of God became a man to enable men to become sons of God.” -- C.S. Lewis.