Youcat commented through CCC - Question n. 84 - Part II.

YOUCAT Question n. 84 - Part II. Was Mary only an instrument of God?

(Youcat answer- repeated) Mary was more than a merely passive instrument of God. The Incarnation of God took place through her active consent as well.      

A deepening through CCC

(CCC 479) At the time appointed by God, the only Son of the Father, the eternal Word, that is, the Word and substantial Image of the Father, became incarnate; without losing his divine nature he has assumed human nature. (CCC 510) Mary "remained a virgin in conceiving her Son, a virgin in giving birth to him, a virgin in carrying him, a virgin in nursing him at her breast, always a virgin" (St. Augustine, Serm. 186, 1: PL 38, 999): with her whole being she is "the handmaid of the Lord" (Lk 1:38).      

Reflecting and meditating 

(Youcat comment) 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”.   

(CCC Comment)

(CCC 511) The Virgin Mary "co-operated through free faith and obedience in human salvation" (LG 56). She uttered her yes "in the name of all human nature" (St. Thomas Aquinas, STh III, 30, 1). By her obedience she became the new Eve, mother of the living.     

(The next question is: Why is Mary our mother also?)