Can a Catholic say that Mary is Co-redeemer? To untangle this theological question, it is necessary to begin from with two fundamental ideas. The first is that God alone can redeem and save us. Redemption entails the pardon of sins and also friendship with the Blessed Trinity – a grace by which God gives himself to man. This is something that only the Lord can grant. From this point of view, man is always the one who needs to receive both pardon and grace.
The second idea is that God accomplishes our redemption by means of the Incarnation, through the life and the Paschal mystery of Christ (his passion, death, and resurrection). And in this matter – the way chosen by God to redeem us – there is place in fact for human cooperation. The very dynamics of the Incarnation show it: “Thus the daughter of Adam, Mary, consenting to the word of God, became the Mother of Jesus. Committing herself wholeheartedly and impeded by no sin to God’s saving will, she devoted herself totally, as a handmaid of the Lord, to the person and work of her Son” (LG 56).
The Tradition of the Church has given also particular relief to Mary's presence at the foot of the cross accompanying her Son. The Second Vatican Council presents the scene this way: “the Blessed Virgin advanced in her pilgrimage of faith, and faithfully persevered in her union with her Son unto the cross, where she stood (cf. Lk. 2:19; 51), in keeping with the divine plan, enduring with her only begotten Son the intensity of his suffering, associated herself with his sacrifice in her mother’s heart, and lovingly consenting to the immolation of this victim which was born of her” (LG 58).
On the basis of this physical and spiritual union of Mary's life with that of Jesus, the Council speaks of a special cooperation of the Virgin in the work of redemption: “in a wholly singular way she cooperated by her obedience, faith, hope, and burning charity in the work of the Savior in restoring supernatural life to souls” (LG 61).
Is it legitimate, therefore, to say, based on these texts or on similar ones, that Mary is Co-redeemer? The response is: it all depends. It is necessary to know for sure how this statement is understood by the one who affirms it or by the one who hears it, because there is a certain ambiguity about the title. In some contexts it may, perhaps, be understood well by all, but that may not always be the case.
A Eucharistic analogy can help us understand the point. For the celebration of Holy Mass the presence of the priest is necessary, and without his presence, Christ's sacrifice cannot be actualized sacramentally. But this does not preclude our saying that the celebration in some way belongs to the whole liturgical assembly as well, to the Christian people present at the Eucharist, for, because of their common priesthood, all those present collaborate with the ministerial action of the priest. To celebrate the Eucharist it is necessary that Christ's action be present (by means of the ministry of the priest) but, simultaneously, the whole assembly "celebrates", as it allows itself to be associated voluntarily with this action of the Lord. Thus, the Eucharist is an action of Christ and of the Church.
In a similar way, one can say that only Christ is the Redeemer and, at the same time, that the Church, associated by Jesus to the Redemption, collaborates in this task: she, the Church, is co-redeemer. The disciples, too, collaborated with Christ in spreading the Gospel. Not only did they preach, they also performed miracles in his name. They were – as St. Paul says – “God's collaborators” (1 Cor 3:9). This kind of collaboration can be applied to the Church and rightly also to Mary. Indeed, she is associated in a special way with the work of redemption, in a manner different from that of the other faithful.
Continuing with the liturgical analogy, it is likewise proper to say that the deacon has a special place in the Eucharistic celebration, alongside the priest, and in a way different from that of the other faithful. United to Christ, by nature [she is his Mother], and in a more intimate, diverse and [supernatural] union than that of other Christians, Mary, too, occupies a special place as she collaborates in the work of redemption.This is something for the theological sciences to discern [and enlarge upon].
Nevertheless, even with these precise observations, a certain ambiguity, which is difficult to avoid, remains in the use of the title of Co-redeemer. The prefix “co” may be interpreted as pointing to an efficient co-causality, and thus, as falling into the error of thinking that the Redemption is a work common to the two [Jesus and Mary]. We believe that the Redemption is the work of Christ alone, since only He is capable of accomplishing it. In this sense (the strict sense), Mary is not a co-redeemer, since she is not, in any way, the “source“ or fount from which the Redemption springs, in a way similar to the deacon who is not a “source“ of the Eucharistic sacrifice; he is not a “concelebrant” in the strict, liturgical sense of the word. This ambiguity is the reason why use of this title has often encountered resistance both among theologians and at the level of the Magisterium, although, once it has been well explained, it is a legitimate title, and has been used sometimes by the Magisterium itself. Those who hold that Mary is not co-redeemer usually do so in order to exclude a way of understanding the faith that not in line with the unique mediation of Christ, influenced, perhaps, as they see it taking shape in some circles, particularly pietistic ones. They are not intent on denying Mary's cooperation in Christ's work of salvation, because this is the ordinary teaching of the Church; they simply want to underline the fact that only Christ can be the source of Redemption and they fear that Mary’s title of Co-redeemer could distort this truth.