19 Yes, truly "blessed is she who believed"! These words, spoken by Elizabeth after the Annunciation, here at the foot of the Cross seem to re-echo with supreme eloquence, and the power contained within them becomes something penetrating. From the Cross, that is to say from the very heart of the mystery of Redemption, there radiates and spreads out the prospect of that blessing of faith It goes right back to "the beginning." and as a sharing in the sacrifice of Christ-the new Adam-it becomes in a certain sense the counterpoise to the disobedience and disbelief embodied in the sin of our first parents. Thus teach the Fathers of the Church and especially St. Irenaeus, quoted by the Constitution Lumen Gentium: "The knot of Eve's disobedience was untied by Mary's obedience; what the virgin Eve bound through her unbelief, the Virgin Mary loosened by her faith." In the light of this comparison with Eve, the Fathers of the Church-as the Council also says-call Mary the "mother of the living" and often speak of "death through Eve, life through Mary." In the expression "Blessed is she who believed," we can therefore rightly find a kind of "key" which unlocks for us the innermost reality of Mary, whom the angel hailed as "full of grace." If as "full of grace" she has been eternally present in the mystery of Christ, through faith she became a sharer in that mystery in every extension of her earthly journey. She "advanced in her pilgrimage of faith" and at the same time, in a discreet yet direct and effective way, she made present to humanity the mystery of Christ. And she still continues to do so. Through the mystery of Christ, she too is present within mankind. Thus through the mystery of the Son the mystery of the Mother is also made clear.
Source: Redemptoris Mater (Vatican.va)