Co-Redemptrix is not dogma. Catholics do not have to believe it. In all truth, it is not really a belief but a truth that some Catholics like to refer to in the term of Co-Redemptirx and others do not.
I think we need to be clearer about the Church’s teaching which is not only in dogmas, but in infallible doctrine, and in non-infallible doctrine.
That Mary is Co-Redemptrix or Mediatrix of all Graces may not be a dogma, but both of these are well-founded teachings of the Catholic Church. Some saints have explicitly used these terms, and Popes (particularly Pope St John Paul II) have clarified their meaning. Pope Emeritus Benedict XVI was right. These terms are not found in the language of the Scriptures and of the Fathers. But so was the word “purgatory” not found. Therefore, while the Church might not elevate these teachings to the level of dogma, it is important that all Catholics realize that these Marian beliefs are solidly founded in Scriptures - provided, of course, that Mary’s role is properly understood as not being equal, but subordinate, to Christ’s work. Just as there is danger in glorifying Mary excessively by use of these titles, there is an equal danger - if we drop the use of these titles altogether - of not giving Mary the recognition that she deserves in the special role that she played in God’s plan of salvation.
Frank J Sheed, that doyen of the Catholic Evidence Guild, in his great Theology And Sanity, Sheed & Ward 1947, expresses the doctrine beautifully:
“As Christ represents humanity in the Redemptive Act, she represents humanity in the co-redemptive act. His suffering was the essential thing, and hers valuable only by derivation. His was the Passion, hers the Com-Passion. He was the Redeemer but the Church loves to call her Co-Redemptrix.” (Theology And Sanity, p 235).
Pope Benedict XV reaffirmed Mary as Co-Redemptrix in his Apostolic Letter Inter soldalica:
“Therefore, one can say, she [that is, the Blessed Virgin] redeemed with Christ the human race.” [Pope Benedict XV, Apostolic Letter *Inter soldalica, AAS 1918, 181].
The Church’s teaching is that the BVM was eternally united with Jesus in the decree for the Incarnation, and thus from eternity before time began and through all eternity (Lumen Gentium 59).
Church Teaching on Mary as Mediatrix of (All) Graces
Compiled by Fr. William G. Most
Just as the pagan may be saved WITHOUT knowing Christ and His teaching or Church specifically, yet every Grace comes to the pagan through Christ’s Church, and through the BVM as Mother of the Church and Mother of Grace.
In the same way as every grace comes to men from Christ through Mary as Mother in the order of grace, so does every grace come to men through Mary as Mother of the Church, for outside of the Church there is no salvation.
“Her status as Mediatrix of All graces exists in a double sense.
“First, she gave the world its Redeemer, the source of all graces, and in this sense she is the channel of all graces.
“Second, Mary is the Mediatrix of all graces because of her intercession for us in heaven. What this means is that no grace accrues to us without her intercession.”
Catholicism and Fundamentalism, Karl Keating, Ignatius, 1988, p 278-279]
Answer by Fr. John Echert on 10-10-2002 (EWTN):
Already the Church regards her as Mediatrix of all grace; it would follow from this that if Mary is a channel of all grace, and since redemption is an act of grace, Mary cooperates completely in the work of Redemption – not as the principal and source, but as channel and partner.