What is the main argument against? Beware of false prophets or those who would argue against (or forget to study) the saints: I think Our Lady being co-redemptrix is consistent with catholic Tradition, especially those "saints of Mary" who explain thus:
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“Together they [Christ and Mary] accomplished the task of man’s redemption…both offered up one and the same sacrifice to God; she in the blood of her heart, he in the blood of the flesh…so that, together with Christ, she obtained a common effect in the salvation of the world.” –Abbot Arnold of Chartres (1144-56), friend and disciple of Saint Bernard of Clairvaux – Doctor of the Church
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So also St. Augustine says: Mary having merited to give flesh to the Divine "Word, and by that to furnish the price of the divine redemption, that we might be delivered from eternal death; therefore is she, says the same doctor, more powerful than any other to help us and obtain for us eternal salvation. Hence St. Theophilus, Bishop of Alexandria, who lived in the time of St. Jerome, thus wrote: The Son is pleased to be entreated by his mother, because he wishes to grant for her sake all that he does grant; aud thus to recompense the favor he has received from her when she gave him flesh. Hence St. John Damascene addresses the Virgin in these words: Thou, then, oh Mary, being mother of God, canst save all men by thy prayers, which are enforced by a mother's authority.
- St Alphonsus, "The Glories of Mary"
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The title Corredemptrix=Coredemptress, which has been current since the fifteenth century, and which also appears in some official Church docuмents under Pius X (cf. D 1978 a), must not be conceived in the sense of an equation of the efficacy of Mary with the redemptive activity of Christ, the sole Redeemer of humanity (I Tim. 2, 5). As she herself required redemption and in fact was redeemed by Christ, she could not of herself merit the grace of the redemption of humanity in accordance with the principle: Principium meriti non cadit sub eodem merito. (The author of an act of merit cannot be a recipient of the same act of merit.)
Her co-operation in the objective redemption is an indirect, remote co-operation, and derives from this that she voluntarily devoted her whole life to the service of the Redeemer, and, under the Cross, suffered and sacrificed with Him. As Pope Pius XII says in the Encyclical “Mystici Corporis” (1943), she “offered Him on Golgotha to the Eternal Father together with the h0Ɩ0cαųst of her maternal rights and her motherly love like a new Eve for all children of Adam” (D 2291). As “The New Eve” she is, as the same Pope declares, in the Apostolic Constitution “Munificentissiumus Deus” (1950) “the sublime associate of our Redeemer” (alma Redemptoris nostri social [cf. Gn. 3, 12]). Cf. D. 3031: generoso Divini Redemptoris socia.
Christ alone truly offered the sacrifice of atonement on the Cross; Mary merely gave Him moral support in this action. Thus Mary is not entitled to the title “Priest” (sacerdos). Christ, as the Church teaches, “conquered the enemy of the human race alone (solus)” (D 711); in the same way, He alone acquired the grace of Redemption for the whole human race, including Mary. The words of Luke I, 38: “Behold the handmaid of the Lord,” imply Mary’s mediate, remote co-operation in the Redemption. Saint Ambrose expressly teaches: “Christ’s Passion did not require any support” (De inst. Virg. 7). In the power of the grace of Redemption merited by Christ, Mary, by her spiritual entering into the sacrifice of her Divine Son for men, made atonement for the sins of men, and (de congruo) merited the application of the redemptive grace of Christ. In this manner she co-operates in the subjective redemption of mankind.
The statement of Pope Pius X in the Encyclical “Ad diem illum” (1904): (Beata Virgo) de congruo, ut aiunt, promeret nobis, quae Christus de condigno promeruit (D 1978 a) (The Blessed Virgin merits for us de congruo what Christ merited de condigno) is, as the present tense ‘promeret” shows, not indeed to be taken as referring to the historical objective Redemption, which occurred once and for all, but to her ever-present, intercessory co-operation in the subjective redemption.
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CHRIST THE SAVIOR
Rev. Reginald Garrigou-Lagrange, O.P.
A Commentary on the Third Part of St. Thomas’ Theological Summa
Seventh Article: The Blessed Virgin Mary’s Universal Mediation
The holy Mother of the Redeemer is often called by the Fathers “the new Eve” or the spiritual mother of all men. Afterward, more and more explicitly her universal mediation was affirmed in the liturgy and in the works of theologians. In the Middle Ages St. Bernard says: “Mary is the procurer of grace, the mediator of salvation, the restorer of the ages,” St. Albert the Great calls Mary “the coadjutor and associate of Christ,” Finally, in most recent times, the Supreme Pontiffs expressly affirm that she is the Mediatrix of all graces.
Leo XIII says: “It is God’s will that nothing be bestowed on us except through Mary; so that, as nobody can reach the supreme Father except through the Son, so that almost nobody can approach Christ except through Mary.” Leo XIII also says: “She is the one from whom Jesus was born, His true Mother, and for this reason the worthy and most accepted Mediatrix to the Mediator.”
Pius X more explicitly declared: “But from the communion of griefs and purpose between Mary and Christ she merited, as Eadmer says, to become most worthily the reparatrix of a lost world, and therefore the dispenser of all the gifts which Jesus procured for us by His death and the shedding of His blood…. Since she excelled all others in sanctity and in her union with Christ and was summoned by Him in the human work of salvation, it was congruous, as they say, that she should merit for us what Christ condignly merited for us; and she is the principal minister in the dispensation of graces.”
Benedict XV likewise says: “As she suffered with her Son in His passion and, so to speak, shared in His death, so she abdicated her maternal rights over her Son for the salvation of men and, as far as it was in her power, sacrificed her Son for the appeasement of divine justice, so that it can truly be said, that along with Christ she redeemed the human race.”
Pius XI said in equivalent words: “The most sorrowful Mother participated in the work of redemption with Jesus Christ.”
Finally, a decree of the Sacred Congregation of the Holy Office praises the custom of attaching the name of Jesus to that of Mary: “His Mother, our co-Redemptress, the blessed Mary.”
Therefore the title “Co-Redemptress of the human race” is approved.
Theological proof. It shows the genuineness of this title, for in the strict sense this title of co-Redemptress and universal Mediatrix befits the Mother of the Redeemer, if she is associated with Christ in the work of the redemption of the human race by way of merit and satisfaction. But she was truly so associated with Him by a perfect communion of will and suffering, inasmuch as she gave her consent to the mystery of the Incarnation. Thus she gave us the Redeemer, and afterward, especially on Calvary, along with Christ congruously merited and satisfied for all of us; now finally in heaven she intercedes with Christ for us and distributes all graces we receive. Therefore the aforesaid title strictly befits her.
But this association with Christ the Redeemer is properly understood when we exclude what it is not. Certainly the Blessed Virgin Mary was not the principal and perfective cause of our redemption, for she could not condignly redeem us in justice. For this, Christ’s theandric act of infinite value, as the head of the human race, was necessary. The Mother of the Savior could not elicit a theandric act of reparation, nor was she constituted the head of the human race. But, subordinated to Christ, she is really the secondary and dispositive cause of our redemption.
It is said “subordinated to Christ” not only in this sense, that she is inferior to Him, but that she concurs in our salvation, by the grace which comes from Christ’s merits. Thus she operated in Him and through Him. Hence Christ is the supreme mediator of all, and the Blessed Virgin Mary was redeemed by Him by a most perfect redemption, not by being freed from sin, but by being preserved from it.
She is also the dispositive cause of our redemption, inasmuch as she disposes us to receive Christ’s influence who, as the author of salvation, perfects the work of our redemption.
Some have raised the objection, that the principle of merit does not come under merit. But the Blessed Virgin Mary was redeemed by the sacrifice of the cross. Therefore she could not even congruously merit the attainment of graces for us.
Reply. I concede the major and minor, but the conclusion does not follow. All that follows is that she could not even congruously merit the attainment of all these graces for herself, this I concede. But she could merit these for us.
Christ merited condignly all the effects of the Blessed Virgin Mary’s predestination, except the divine motherhood, because in such a case He would have merited the Incarnation and therefore Himself. Hence Christ merited the first grace and final perseverance for the Blessed Virgin Mary. But the Blessed Virgin Mary did not even congruously merit for herself either the first grace or final perseverance, because the principle of merit does not come under merit. But the Blessed Virgin Mary merited for us congruously what Christ merited for us condignly, namely, all the graces we receive, even the first grace and final perseverance. In this there is no contradiction, but great harmony.
Hence the Blessed Virgin Mary was indeed redeemed by Christ through the sacrifice of the cross in the preservative sense, and so she was immaculate; but as a consequence of this, she merited congruously with Christ for us, not only the distribution or application of graces, but the attainment of graces that flow from the sacrifice of the cross; for in the strict sense together with Christ she offered this sacrifice. Thus she merited with Him redemption in the objective sense, namely, our liberation from sin and our reinstatement in grace.
But I insist. The Blessed Virgin Mary merited congruously for us what, for example, St. Monica congruously merited and obtained for St. Augustine, namely, the grace of conversion. Therefore there is only a difference of degree between her and other saints who intercede for us, and it must not be said that she is the Co-Redemptrix in the strict sense, but only in an improper sense, as the apostles are said to have labored for the salvation of souls.
Reply. The difference is that the Blessed Virgin Mary gave us the Redeemer, and with Him offered the sacrifice of the cross by meriting and satisfying. St. Monica and other saints, on the contrary, did not offer with Christ the sacrifice of the cross, and therefore did not merit congruously the attainment of graces that flow from this sacrifice but only the application of these, and therefore cannot be called co-redeemers. They can be said only to labor in the salvation of souls. They did not merit congruously our redemption in the objective sense.
Hence St. Albert the Great could say that the Blessed Virgin Mary is not assumed into the ministry of our Lord, but as a consort and help, in accordance with the saying: “Let us make him a help like unto himself, ” (Gen. 2:18). In this the Blessed Virgin is above the apostles and she alone can be properly called the Mediatrix and co-redemptrix of the human race.
Yes. Mother Mary is the Co-Redemptrix. The Dimonds are wrong. Go with St. Padre Pio, St. Mother Theresa, St. Maximillian Kolbe, St. Alphonsus and St. Bernard on this, rather than the Dimonds. Excellent quotes from Pax. Numerous Popes have taught it too, so we know for certainty the doctrine is at least infallibly safe and cannot be heretical. It is not yet defined, though it should probably become the Fifth Marian Dogma, along with Mother Mary being recognized as Advocate and Mediatrix of All Graces. Here is a Petition:
https://marianapostolate.com/petition-for-the-papal-definition-of-mary-coredemptrix-mediatrix-and-advocate/
"Your Holiness:
In a spirit of filial love and obedience, we, as members of the People of God, wish to humbly bring before you our petition and prayer for the solemn papal definition of the revealed role of Mary most holy as the Spiritual Mother of all peoples under its three principal aspects as Co-redemptrix, Mediatrix of all graces, and Advocate.
We firmly believe, Holy Father, that the solemn papal declaration of the Blessed Virgin Mary as the Spiritual Mother of all humanity in her roles which God has given her as Co-redemptrix, Mediatrix of all graces, and Advocate will bring great graces to the Church and the world by an explicit solemn recognition of her maternal role on the part of the Church, and thus allowing her to exercise fully the motherly gift which Jesus Christ gave to humanity from the cross: “Woman, behold, your son!… Behold, your mother!” (Jn. 19:26-27). We believe that this dogmatic proclamation will also further the authentic ecuмenical mission of the Church by proclaiming the revealed truth about Mary, who collaborated in an altogether unique way in the work of our redemption in a manner that was completely subordinate to and dependent upon Jesus Christ, the one divine Redeemer of the human race.
In view of the many serious crises presently facing the entire human family, including war, terrorism, moral decline, and natural disaster, we humbly request that you solemnly declare the dogma of Mary as the Spiritual Mother all peoples, specifying that she is the Co-redemptrix, Mediatrix of all graces, and Advocate, and thus to provide for the full actuality of her motherly roles of intercession for humanity, which we believe will effect a profound historic and continuing grace for the Church and for the world.