About Mater Ecclesiae
When reading the Wikipedia article Mother of the Church (Mater Ecclesiae), I was surprised how ancient the title was, and how it was only rediscovered in 1947 by a German Jesuit historian Hugo Rahner, which led to Pope Paul VI and his successors to start using the title:
The use of the Mater Ecclesiae title to the Virgin Mary goes back to Ambrose of Milan in the 4th century, but this was not known until its 1944 rediscovery by Hugo Rahner. Rahner's Mariology, following Ambrose, sees Mary in her role within the Church. His interpretation, based solely on Ambrose and the early Fathers, greatly influenced Vatican II and Pope Paul VI, who, quoting Ambrose, declared Mary the "Mother of the Church".
Pope John Paul II's teaching
Pope John Paul II explained the meaning and the significance of the title in his 1987 encyclical Redemptoris Mater paragraph 47:
At the Council Paul VI solemnly proclaimed that Mary is the Mother of the Church, "that is, Mother of the entire Christian people, both faithful and pastors."134 Later, in 1968, in the Profession of faith known as the "Credo of the People of God." he restated this truth in an even more forceful way in these words: "We believe that the Most Holy Mother of God, the new Eve, the Mother of the Church, carries on in heaven her maternal role with regard to the members of Christ, cooperating in the birth and development of divine life in the souls of the redeemed."135
The Council's teaching emphasized that the truth concerning the Blessed Virgin, Mother of Christ, is an effective aid in exploring more deeply the truth concerning the Church. When speaking of the Constitution Lumen Gentium, which had just been approved by the Council, Paul VI said: "Knowledge of the true Catholic doctrine regarding the Blessed Virgin Mary will always be a key to the exact understanding of the mystery of Christ and of the Church."136 Mary is present in the Church as the Mother of Christ, and at the same time as that Mother whom Christ, in the mystery of the Redemption, gave to humanity in the person of the Apostle John. Thus, in her new motherhood in the Spirit, Mary embraces each and every one in the Church, and embraces each and every one through the Church. In this sense Mary, Mother of the Church, is also the Church's model. Indeed, as Paul VI hopes and asks, the Church must draw "from the Virgin Mother of God the most authentic form of perfect imitation of Christ."137
Thanks to this special bond linking the Mother of Christ with the Church, there is further clarified the mystery of that "woman" who, from the first chapters of the Book of Genesis until the Book of Revelation, accompanies the revelation of God's salvific plan for humanity. For Mary, present in the Church as the Mother of the Redeemer, takes part, as a mother, in that monumental struggle; against the powers of darkness"138 which continues throughout human history. And by her ecclesial identification as the "woman clothed with the sun" (Rev. 12:1),139 it can be said that "in the Most Holy Virgin the Church has already reached that perfection whereby she exists without spot or wrinkle." Hence, as Christians raise their eyes with faith to Mary in the course of their earthly pilgrimage, they "strive to increase in holiness."140 Mary, the exalted Daughter of Sion, helps all her children, wherever they may be and whatever their condition, to find in Christ the path to the Father's house.
Thus, throughout her life, the Church maintains with the Mother of God a link which embraces, in the saving mystery, the past, the present and the future, and venerates her as the spiritual mother of humanity and the advocate of grace.
Pope Francis's decree
On February 11, 2018, Pope Francis signed a decree to insert The Memorial of the Blessed Virgin Mary, Mother of the Church into the Roman Calendar on the Monday following Pentecost Sunday. Text of the decree here.
Question and purpose
Do we have the writings by St. Ambrose or quotations from them in which the title first appeared?
One clue is to look for it in Hugo Rahner's 1944 book: Mater Ecclesia: Lobpreis der Kirche aus dem ersten Jahrtausend christlicher Literatur.
The Wikipedia article also noted that the title is also found in the writings of Berengaud, bishop of Treves (d. 1125). I wonder how the concept of Mater Ecclesiae developed from 4th century St. Ambrose (AD c. 339 - c. 397) to 12th century Berengaud, bishop of Treves (d. 1125) to 20th century Pope John Paul II's Redemptoris Mater to 21st century Pope Francis I's decree.
I would like to know the original context in St. Ambrose's writing (how he understood the title) and whether his meaning was consistent with how Pope John Paul II explained it, and how much of an expansion of meaning was it across the ages.