The following, taken from a theological work defending the fifth Marian doctrine (Mary as co-redemptrix and co-mediatrix), highlights why theological ecumenism with Roman Catholicism will forever be an impossibility for Latter-day Saints while Rome preaches as doctrine (and perhaps soon, dogma) this teaching, as well as other false doctrines about Mary such as the Immaculate Conception and the Eucharistic Sacrifice of the Mass:
It is in view of the merits of Christ that his heavenly Father is willing to be reconciled to mankind, to forget out sins, to pour out His grace abundantly upon us. It is when He beholds the Divine Victim, that has been slain for us, and sees His wounds, and hears the voice of His Blood crying out for pardon for His brethren, that the heart of the Father is moved, that the gates of His mercy are unlocked, and that the streams of grace flow downward to bathe the earth, and render fruitful the barren hearts of men.
But the Lamb of God, immolated on Calvary, was offered up through Mary’s hands. He did indeed offer Himself as the principal agent, even as in Holy Mass it is He who is High Priest no less than Victim. But as in the Eucharistic Sacrifice He is pleased to use as His instrument a moral man, so too it was His will, that, as He hung upon the Cross, His Mother should be present as His side, that by the union of her will with His, she might cooperate with Him in His saving oblation.
And so, even now in Heaven, while no gift is bestowed on us by God except in virtue of the Passion and death of His Divine son, the Precious Blood, the sacred wounds of Jesus, must be presented to God by Mary, and, apart from her cooperation, no grace descends to earth, no new from Heaven refreshes the human heart, no bounteous rain, dropping on it from above, makes it fruitful in salutary works.
In this the mediation of our Blessed Lady mainly consists. She does, indeed, intercede for us in our varied needs, and her own extraordinary grace and merit, her own wondrous prerogatives, give to her prayer an irresistible force and efficacy. She is the Mother of Jesus, she is the glorious daughter of the Most High, she is the cherished Spouse of the Holy Ghost, and as such she can never meet with rebuff. Her slightest wish carries with it a resistless appeal, to which the Lord of all things can never be insensible. Yet more powerfully still than ever Mary’s exalted merit does the Precious Blood of Jesus plead for us, and whatever power her prayers possess to render God propitious to the sinner, all comes to them from the Passion of Christ, her Son.
On the other hand, it has been the will of God, that from the first to last Jesus, as the Victim of atonement for our sins, should be presented to Him by Mary. It was so when the Virgin Mother gave her consent to the divine plan of Redemption, as declared to her by the Angel at Nazareth. It was so when she ratified her consent at the birth of the Saviour at Bethlehem, at His Presentation in the temple at Jerusalem, and finally as she stood beneath the Cross on Calvary. (Raphael V. O’Connell, S.J. [Baltimore: Metropolitan Press, 1926], 110-12; emphasis added)
