Mary is "the figure of the Church, who is immaculate yet espoused"

Where do we find the first mention of the expression "Mary, Mother of the Church"? Two Church Fathers, St. Augustine and St. Leo the Great, give us the first glimpses of it. The first affirms that Mary is "the mother of the members of Christ"; the second says: "The whole community of the faithful was begotten with Christ at the Nativity."

Bishop Dominique Le Tourneau, author of the Encyclopedic Dictionary of Mary, explains: "This community of the faithful is none other than the Church, who was born from Mary's womb when her head, the Christ, was born." He also cites St. Hilary of Poitiers who, in the fourth century, "emphasizes the link between the birth of Christ from the Virgin Mary and the spiritual birth of Christians": "The Church has her origin in Bethlehem, for she began to exist in Christ" (Tractatus super Psalmos).

For St. Ambrose, the Church was mystically born from the Virgin's womb when she gave birth to the incarnate Word. "It is rightly said that [Mary] was married and was a virgin, for she was the figure of the Church, who is immaculate, yet espoused. The Virgin conceived us spiritually, and the Virgin brought us into the world without groaning" (De institutione virginis).

The first person to use the title "Mother of the Church" specifically, according to Bishop Le Tourneau, seems to be a certain Berengaud de Ferrières, a Benedictine monk of the 9th century.

After these, the popes issued their own statements. "The Catholic Church, instructed by the magisterium of the Holy Spirit, has always professed the highest devotion to Mary, as the most loving Mother, who was left as an inheritance by the very voice of Jesus, her dying spouse," Benedict XIV wrote in his bull Gloriosæ Dominæ, in 1748. Then Leo XIII (1878-1903) declared that "she showed herself to be truly Mother of the Church and was truly Teacher and Queen of the Apostles." In 2009, Benedict XVI himself mentioned this title. 

Excerpt from: Famille Chrétienne