40 After the events of the Resurrection and Ascension Mary entered the Upper Room together with the Apostles to await Pentecost, and was present there as the Mother of the glorified Lord. She was not only the one who "advanced in her pilgrimage of faith" and loyally persevered in her union with her Son "unto the Cross," but she was also the "handmaid of the Lord," left by her Son as Mother in the midst of the infant Church: "Behold your mother." Thus there began to develop a special bond between this Mother and the Church. For the infant Church was the fruit of the Cross and Resurrection of her Son. Mary, who from the beginning had given herself without reserve to the person and work of her Son, could not but pour out upon the Church, from the very beginning, her maternal self-giving. After her Son's departure, her motherhood remains in the Church as maternal mediation: interceding for all her children, the Mother cooperates in the saving work of her Son, the Redeemer of the world. In fact the Council teaches that the "motherhood of Mary in the order of grace...will last without interruption until the eternal fulfillment of all the elect." With the redeeming death of her Son, the maternal mediation of the handmaid of the Lord took on a universal dimension, for the work of redemption embraces the whole of humanity. Thus there is manifested in a singular way the efficacy of the one and universal mediation of Christ "between God and men" Mary's cooperation shares, in its subordinate character, in the universality of the mediation of the Redeemer, the one Mediator. This is clearly indicated by the Council in the words quoted above. "For," the text goes on, "taken up to heaven, she did not lay aside this saving role, but by her manifold acts of intercession continues to win for us gifts of eternal salvation." With this character of "intercession," first manifested at Cana in Galilee, Mary's mediation continues in the history of the Church and the world. We read that Mary "by her maternal charity, cares for the brethren of her Son who still journey on earth surrounded by dangers and difficulties, until they are led to their happy homeland." In this way Mary's motherhood continues unceasingly in the Church as the mediation which intercedes, and the Church expresses her faith in this truth by invoking Mary "under the titles of Advocate, Auxiliatrix, Adjutrix and Mediatrix."
Source: Redemptoris Mater (Vatican.va)