From YOUCAT, fact 84:
84: Was Mary only an instrument of God?
Mary was more than a merely passive instrument of God. The Incarnation of God took place through her active consent as well. [493-494, 508-511]
When the angel told her that she would bear “the Son of God,” Mary replied, “Let it be to me according to your word” (Lk 1:38). The redemption of mankind by Jesus Christ thus begins with a request by God and the free consent of a human being – and a pregnancy before Mary was married to Joseph. By such an unusual path Mary became for us the “Gate of Salvation.”