Here are tidbits from an article Compiled by Martin Beckman
Mary as Coredemptrix: An Explanation
When the Church invokes Mary under the title, “Coredemptrix”, she means that Mary uniquely participated in the redemption of the human family by Jesus Christ, Our Lord and Saviour. At the Annunciation (cf.Lk.1:38) Mary freely cooperated in giving the Second Person of the Trinity his human body which is the very instrument of redemption, as Scripture tells us: “We have been sanctified through the offering of the body of Jesus Christ once for all” (Heb.10:10).
And at the foot of the cross of our Saviour (Jn.19:26), Mary’s intense sufferings, united with those of her Son, as Pope John Paul II tells us, were, “also a contribution to the Redemption of us all” (Salvifici Doloris, n.25). Because of this intimate sharing in the redemption accomplished by the Lord, the Mother of the Redeemer is uniquely and rightly referred to by Pope John Paul II and the Church as the “Coredemptrix.”
It is important to note that the prefix “co” in the title Coredemptrix does not mean “equal to” but rather “with”, coming from the Latin word cum. The Marian title Coredemptrix never places Mary on a level of equality with her Divine Son, Jesus Christ. Rather it refers to Mary’s unique human participation which is completely secondary and subordinate to the redeeming role of Jesus, who alone is true God and true Man.
Mary’s role was unique. If she had said ‘no’ to Gabriel … to God, would we have a Savior, would we have our true Redeemer … our Lord … the Messiah? Mary played a definite role in our salvation. But back to the original statement … that role is entirely dependent and subordinate on Jesus.
Mary is called to give her free and full consent to conceive this child. She is not merely a passive recipient of the message, but she was given an active role, and heaven awaited her free choice. It is precisely by her free consent to collaborate in God’s saving plan that she becomes the Coredemptrix. The prophecy of Simeon to Mary, “and a sword will pierce through your own soul also” (Luke 2:25), affirms Mary’s unique participation in the work of redemption, as it warns her that she will undergo an unspeakable pain that will pierce her soul, for the salvation of mankind. John 19:25 tells us of Jesus’ Mother at the very foot of the cross, persevering with her Son in his worst hour of agony, and therein suffering the death of her Son.
Thus in her own suffering too, the Mother of the Redeemer participates in the redemptive mission of Jesus Christ.
St. Paul tells us we are to make up what is lacking in the sacrifice of Jesus (Col 1:24): “Now I rejoice in my sufferings for your sake, and in my flesh I complete what is lacking in Christ’s afflictions for the sake of his body, that is, the church,”
Paul is making a very similar statement here also. By his sufferings he is completing what is lacking in Christ’s afflictions for the church and us. This is a role we all can partake … but this role is dependent on Christ and subordinate to Christ.
That is all that statement about Mary is saying. Mary had a role, a contribution in filling what was lacking in us, the Church. It’s a very biblical statement.
Jesus Christ as true God and true man redeems the human family, while Mary as Coredemptrix participates with the Redeemer in his one perfect Sacrifice in a completely subordinate and dependent way. The key word here is “participation” in that which is exclusively true of Jesus Christ. The title “Coredemptrix” never puts Mary on a level of equality with our Lord; rather, it refers to Mary’s unique and intimate participation with her divine Son in the work of redemption. “Coredemptrix” is a Latin word; the prefix “co” in the title, “Coredemptrix,” derives from the Latin word “cum,” which means “with,” not “equal to.” Mary’s sufferings are efficacious towards the redemption of man because they are wholly rooted in the redemptive graces of Christ and are perfectly united to His redeeming will. Similarly, as Mediatrix, the Mother of Jesus does not “rival” Christ’s mediation but rather participates in the one mediation of Jesus Christ. Imagine water from a reservoir reaching the people through a system of aqueducts or channels. By analogy, Jesus is the infinite “reservoir” of all grace, which is distributed to us through Mary … as she gave birth to Jesus. Jesus, the one mediator, does not exclude secondary, subordinate mediators.
Catholics do agree wholeheartedly that Jesus is the one and only mediator between man & God. No question … the bible teaches this … the Catholic Church teaches this. No subordinate co-deities, no additional redeemers, no additional mediators! Clear enough?
But what about our role in bringing people to Christ, preaching the Gospel, as teachers, pointing people to Christ … and so on? We can be mediators in that fashion. Surely you do not disagree that faith comes from (by grace) from receiving the gospel message.
This is not saying we are mediators between Jesus and God for mankind … but we can have a subordinate & dependent role.
This isn’t adding to Jesus’ mediatorship, not a seperate channel, not an end-run, or anything that takes away from His role.