Wikipedia
Co-Redemptrix is a title used by some Roman Catholics for the Blessed Virgin Mary, as well as a Catholic theological concept referring to Mary's role in the redemption of man. It has always been controversial and has never formed part of the dogma of the Church. The term Co-Redemptrix refers to a subordinate but essential participation by the Blessed Virgin Mary in redemption, notably that she gave free consent to give life to the Redeemer, to share his life, to suffer with him under the cross, to offer his sacrifice to God the Father for the sake of the redemption of mankind. Related to this belief is the concept of Mary as Mediatrix, which is a separate concept but regularly included by faithful who use the title Co-Redemptrix.
The concept was especially common in the late Middle Ages, when it was promoted heavily among the Franciscans, and often resisted by the Dominicans. By the early 16th century the hopes of the concept becoming Catholic doctrine had receded, and have never seriously revived. In more recent times, the title has received some support from the Catholic Magisterium though it is not included in the concluding chapter of the apostolic constitution Lumen gentium of the Second Vatican Council, which chapter many theologians hold to be a comprehensive summary of Roman Catholic Mariology. Some, in particular the adherents of the Amsterdam visions, have petitioned for a dogmatic definition, along with Mediatrix, but recent high-level comments in the Church have not encouraged these hopes.
The concept of co-redemption is not new. Even before the year 200, the Church Father Irenaeus referred to Mary as the cause of our salvation given her "fiat" It is a concept which was the subject of considerable theological debate, reaching a peak in the 15th century, but attempts to have it declared a dogma were not successful. In general it was supported and promoted by medieval Franciscans and opposed by Dominicans.
A number of theologians have discussed the concept over the years, from the 19th century Father Frederick William Faber, to the 20th century Mariologist Father Gabriel Roschini. In his 1946 publication Compendium Mariologiae, Roschini explained that Mary did not only participate in the birth of the physical Jesus, but, with conception, she entered with him into a spiritual union. The divine salvation plan, being not only material, includes a permanent spiritual unity with Christ. Most Mariologists agree with this position. Mary suffered willingly under the cross and, in a sense, offered Christ's sacrifice to the God the Father.
The concept of Mary offering Christ's sufferings is theologically complex. Christ offered himself alone; “the Passion of Christ did not need any assistance.” It is according to the spirit of the offertory within the Mass, in which those assisting in the sacrifice bring their particular offerings, personal hardships, etc., and offer them to Christ to be included in his sacrifice, inasmuch as they belong to his mystical body; just the same, they also offer Christ's own sacrifice, acknowledging the paucity of their own offerings and that not even the greatest effort, of itself and apart from Christ, can be of any significance to God. A priest is, in a sense, able to participate in the sacrifice in a sacramental manner. The Holy Office has forbidden reference to Mary as a priestess.
Mary “ merits for us de congruo”, i.e. by way of a fitting reward not binding upon God, “what Jesus Christ merits for us de condigno”, i.e. by God binding himself to give the reward. It is uncertain whether Pope Pius X meant "for us” to mean all mankind except Christ's human nature and Mary, or only those living after Mary's merits, since the former could potentially break the general rule that the effect comes after the cause. Where it concerns post- Assumption graces, it is a pious opinion that the entirety of them is effected not without an intercession of Mary.
The Roman Catholic view of the title Co-Redemptrix does not imply that Mary participates as equal part in the redemption of the human race, since Christ is the only redeemer. Mary herself needed redemption and was redeemed by Jesus Christ. Being redeemed by Christ, implies that she cannot be his equal part in the redemption process. Additionally, Mary intercedes for all graces that are given, this is not because God needed her intercession in any way to give them; rather, it “is to be so understood that it neither takes away from nor adds anything to the dignity and efficaciousness of Christ the one Mediator”.
Pope Pius XII has thus in Munificentissimus Deus, the bull defining the dogma of the Assumption of Mary, used the alternative expression "the revered Mother of God, [...] joined [...] with Jesus Christ in one and the same decree of predestination [...] as the noble associate of the divine Redeemer".