God's Decree & Mary's Faith

The Response of Mary's Faith established the means which God would use to fulfill His Decree.
You will notice that Mary does not dispute God's purpose; what He has decreed will come to pass. She does not fight with what God has decreed. She simply cannot understand how she, a virgin, could conceive a child without knowing a man. This was not unbelief on her part, as it was on Zacharias' part when he was told by Gabriel that he and Elizabeth were going to have a child. He did not believe Gabriel's words to him (vs. 18) and he was struck mute by Gabriel on account of it, until the day that John the Baptist was born.
In Mary's case, there was not that unwillingness to believe as there was in Zacharias. Quite the contrary. She was receiving his word to her. And so Gabriel gives her the explanation; that the Holy Spirit will come upon her, and the power of the highest would overshadow her, and therefore, that Holy One who would be born would be called the Son of God. Then he also tells her, to encourage her faith, that her relative Elizabeth had conceived a child in her old age; she who was called barren. For with God nothing will be impossible.