An excerpt from the writing of Rev. Fr. Reginald Garrigou-Lagrange, O.P.
Evidently Mary is not our mother in the ordinary sense of the term since she did not give us natural life. Considering our natural life, it is Eve who deserves to be called the mother of all men. Mary is our mother rather in a spiritual sense through adoption, for, by her union with Jesus the Redeemer, she has communicated to us the supernatural life of grace. She is very much more than a sister in grade: we say, on the analogy of natural life, that she has given us birth to a divine form of life. St. Paul could say, speaking to the Corinthians, 'In Christ Jesus, by the gospel, I have begotten you'. With still more truth can we speak of Mary's spiritual maternity - a maternity which is source of a life destined to endure not sixty or eighty years, but all eternity.
Mary's maternity is adoptive, as is God's fatherhood of the just. It is however, much more intimate and fruitful than in ordinary human adoption. Human adoption constitutes a person legally the child and heir of another. But all this is in the legal order; and even though it is a sign of the affection bestowed on the adopted child, it does not produce any interior change in it. Divine adoption, on the contrary, produces sanctifying grace in the soul of the just, therby making it to participate in the divine nature and to have within itself the germ of eternal life.