10 For is not Mary the Mother of Christ? Then she is our Mother also. And we must in truth hold that Christ, the Word made Flesh, is also the Savior of mankind. He had a physical body like that of any other man: and again as Savior of the human family, he had a spiritual and mystical body, the society, namely, of those who believe in Christ. "We are many, but one sole body in Christ" ( Rom . xii., 5). Now the Blessed Virgin did not conceive the Eternal Son of God merely in order that He might be made man taking His human nature from her, but also in order that by means of the nature assumed from her He might be the Redeemer of men. For which reason the Angel said to the Shepherds: "To-day there is born to you a Savior who is Christ the Lord" ( Luke ii., 11). Wherefore in the same holy bosom of his most chaste Mother Christ took to Himself flesh, and united to Himself the spiritual body formed by those who were to believe in Him. Hence Mary, carrying the Savior within her, may be said to have also carried all those whose life was contained in the life of the Savior. Therefore all we who are united to Christ, and as the Apostle says are members of His body, of His flesh, and of His bones ( Ephes . v., 30), have issued from the womb of Mary like a body united to its head. Hence, though in a spiritual and mystical fashion, we are all children of Mary, and she is Mother of us all. Mother, spiritually indeed, but truly Mother of the members of Christ, who are we (S. Aug. L. de S. Virginitate, c. 6).
Source: Ad Diem Illum Laetissimum (Vatican.va)