God chose Mary to play an extraordinary role in His plan to save the world. She was chosen to conceive and bear the Messiah who was/is Jesus of Nazareth. The conception of Jesus in the womb of Mary required a supernatural work for she was a virgin. The angel informed Joseph, “…that which is conceived in her is of the Holy Spirit.” (See Matthew 1:20.) In answer to Mary’s question, “How can this be since I do not know a man?” the angel said, “The Holy Spirit will come upon you, and the power of the Highest will overshadow you; therefore, also, that Holy One who is to be born will be called the Son of God.” (See Luke 1:34-35.)
Mary believed this incredible promise though it went against everything she knew about reproduction (Luke 1:34-45, esp. verses 38, 45). In the context, Mary should be contrasted with Zacharias who did not believe the promise that his barren wife would have a son (1:5-25, esp. verses 18-20). Do you believe all the promises God has made to you though sometimes it is difficult to do so given the circumstances?