Calling Mary a "Co-redemptrix" places her on an equal level with Jesus Christ, the Divine Son of God, making her something like a fourth person of the Trinity, a goddess or quasi-divine goddess, which is blasphemy for any true Christian.
The Catholic Church's use of the title, "Co-redemptrix" as applied to the Mother of Jesus in no sense places Mary on a level of equality with Jesus Christ, the Divine Redeemer. There is an infinite difference between the divine person of Jesus Christ and the human person, Mary. Rather, papal teaching has used the title, "Co-redemptrix" to refer to the unique participation of the Mother of Jesus with and under her divine son in the work of human redemption.
The term, "co-redemptrix" is properly translated "the woman with the redeemer" or more literally "she who buys back with (the redeemer)." The prefix, "co," comes from the Latin term, "cum," which means "with" and not "equal to." Co-redemptrix therefore as applied to Mary refers to her exceptional cooperation with and under her divine son, Jesus Christ in the redemption of the human family, as manifested in Christian Scripture.
With Mary's free and active "fiat" to the invitation of the angel Gabriel to become the mother of Jesus, "Be it done unto me according to your word" (Lk. 1:38), she uniquely cooperated with the work of redemption by giving the divine Redeemer his body, which was the very instrument of human redemption. "We have been sanctified by the offering of the body of Jesus Christ once for all" (Heb. 10:10), and the body of Jesus Christ is given to him through the free, active, and unique cooperation of the Virgin Mary. By virtue of giving flesh to the "Word made flesh" (Jn.1:14), who in turn redeems humanity, the Virgin of Nazareth uniquely merits the title Co-redemptrix. In the words of the late Mother Teresa of Calcutta, "Of course Mary is the Co-redemptrix—she gave Jesus his body, and his body is what saved us." (1)
The New Testament prophecy of Simeon in the temple also reveals the suffering, co-redemptive mission of Mary in direct union with her Redeemer son in their one unified work of redemption: "Simeon blessed them, and said to Mary, his mother, 'Behold, this child is set for the fall and rise of many in Israel, and will be a sign of contradiction, and a sword shall pierce through your own soul, too" (Lk. 2:34-35).
But the climax of Mary's role as Co-redemptrix under her divine son takes place at the foot of the Cross, where the total suffering of the mother's heart is obediently united to the suffering of the Son's heart in fulfillment of the Father's plan of redemption (cf. Gal 4:4). As the fruit of this redemptive suffering, Mary is given by the crucified Savior as the spiritual mother of all peoples, "Woman, behold your son!' Then he said to the disciple, 'behold, your mother!" (Jn.19:27). As described by Pope John Paul II, Mary was "spiritually crucified with her crucified son" at Calvary, and "her role as Co-redemptrix did not cease with the glorification of her Son." (2) Even after the accomplishment of the acquisition of the graces of redemption at Calvary, Mary's co-redemptive role continues in the distribution of those saving graces to the hearts of humanity.
The earliest Christian writers and Fathers of the Church explained Marian co-redemption with great profundity in simplicity in the first theological model of Mary as the "New Eve." Essentially, they articulated that as Eve, the first "mother of the living" (Gen. 3:20) was directly instrumental with Adam, the father of the human race, in the loss of grace for all humanity, so too Mary, the "New Eve," was directly instrumental with Jesus Christ, whom St. Paul calls the "New Adam" (Cf. 1 Cor. 15:45-48), in the restoration of grace to all humanity. In the words of 2nd century Church Father, St. Irenaeus: "Just as Eve, wife of Adam, yet still a virgin, became by her disobedience the cause of death for herself and the whole human race, so Mary, too, espoused yet a virgin, became by her obedience the cause of salvation for herself and the whole human race." (3)
In light of her unique and direct cooperation with the Redeemer in the restoration of grace for the human family (cf. Gen. 3:15), Mary became universally known in the early Church as the "New Mother of the Living," and her instrumental co-redemption with Christ was well summed in the succinct expression of 4th century Church Father, St. Jerome: "Death through Eve, life through Mary." (4)
Explicit references to Marian co-redemption as Mary's unique participation with and under Jesus Christ in redeeming or "buying back" humanity from the slavery of Satan and sin is present throughout Christian Tradition. For example, the 7th century Church writer, Modestus of Jerusalem, states that through Mary, we "are redeemed from the tyranny of the devil." (5) St John Damascene (8th century) greets her: "Hail thou, through whom we are redeemed from the curse." (6) St. Bernard of Clairvaux (12th century) preaches that, "through her, man was redeemed." (7) The great Franciscan Doctor, St. Bonaventure (13th century) aptly summarizes Christian Tradition in this teaching: "That woman (namely Eve), drove us out of Paradise and sold us; but this one (Mary) brought us back again and bought us." (8)
Although there was never any question of the total and radical dependency of the Virgin Mary's participation in redemption upon the divine work and merits of Jesus Christ in the minds of the Church fathers and doctors, nonetheless early Christian Tradition did not hesitate to teach and preach the unparalleled intimate participation of the woman, Mary, in the "buying back" or redeeming of the human race from the slavery of Satan. As humanity was sold by a man and a woman, so it was God's will that humanity would be bought back by a Man and a woman.
It is upon this rich Christian foundation that 20th century popes and saints have used the title Co-redemptrix for Mary's unique role in human redemption, as exemplified in the contemporary use of Co-redemptrix for Mary by Pope John Paul II on at least six occasions during his present pontificate. (9) "Co-redemptrix" as used by the popes means no more that Mary is a goddess equal with Jesus Christ than St. Paul's identification of all Christians as "God's co-workers"(1 Cor. 3:9) means that Christians are gods equal to the one God.
All Christians are rightly called to be co-workers or "co-redeemers" with Jesus Christ (cf. Col. 1:24) in the reception and cooperation with grace necessary for our own redemption and the redemption of others—personal subjective redemption made possible by the historic objective redemption or "buying back" accomplished by Jesus Christ, the "New Adam," the Redemptor, and Mary, the "New Eve," the Co-redemptrix.