Ever since St. Ambrose of Milan there has been a line of thinking that Mary is the "Mother of the Church" - what is to be understood by this title?
Catechism of the Catholic Church 963 referencing Lumen Gentium 53 says, "She is 'clearly the mother of the members of Christ' . . . since she has by her charity joined in bringing about the birth of believers in the Church, who are members of its head."
Lumen Gentium 53 The Virgin Mary, who at the message of the angel received the Word of God in her heart and in her body and gave Life to the world, is acknowledged and honored as being truly the Mother of God and Mother of the Redeemer. Redeemed by reason of the merits of her Son and united to Him by a close and indissoluble tie, she is endowed with the high office and dignity of being the Mother of the Son of God, by which account she is also the beloved daughter of the Father and the temple of the Holy Spirit. Because of this gift of sublime grace she far surpasses all creatures, both in heaven and on earth. At the same time, however, because she belongs to the offspring of Adam she is one with all those who are to be saved. She is "the mother of the members of Christ . . . having cooperated by charity that faithful might be born in the Church, who are members of that Head."1 Wherefore she is hailed as a pre-eminent and singular member of the Church, and as its type and excellent exemplar in faith and charity. The Catholic Church, taught by the Holy Spirit, honors her with filial affection and piety as a most beloved mother.
1. cf. Rom 8:29.