The Blessed Virgin Mary has been given the title of Mother of the Church since she gave birth to Christ, the Head of the Church, and she became the Mother of the redeemed people before her Son had given up the spirit on the Cross. Pope Paul VI solemnly confirmed the title in an address to the Fathers of the Second Vatican Council on 21 November 1964 and decreed that the whole Christian people should, by the use of this beautiful title, give still greater honour to the Mother of God.
‘The joyous veneration given to the Mother of God by the contemporary Church, in light of reflection on the mystery of Christ and on his nature, cannot ignore the figure of a woman (cf. Gal 4:4), the Virgin Mary, who is both the Mother of Christ and Mother of the Church. In some ways this was already present in the mind of the Church from the premonitory words of Saint Augustine and Saint Leo the Great. In fact the former says that Mary is the mother of the members of Christ, because with charity she cooperated in the rebirth of the faithful into the Church, while the latter says that the birth of the Head is also the birth of the body, thus indicating that Mary is at once Mother of Christ, the Son of God, and mother of the members of his Mystical Body, which is the Church’ (Decree of the Sacred Congregation for Divine Worship)
Taken from Universalis.