The Virgin Mary is Mediatrix of Grace. Pope Pius XI even calls her “Mediatrix of all graces.” However, this phrasing is inexact. The Pope was not referring to all graces without exception, but to all graces to us, to fallen humanity. A correct understanding requires two exceptions to the phrase “Mediatrix of all graces.”
1. “The Virgin Mary is not the Mediatrix of the graces which flow from the Divine Nature of Christ to the human nature of Christ. There can be no mediator between Christ’s Divine Nature and his human nature; therefore, there can be no Mediatrix between Christ’s Divine Nature and his human nature. For the role of the Mediatrix is to assist the Mediator. Where there is no mediation, there is no role for the Mediatrix.
“The Virgin Mary does not intercede, within the hypostatic union, between the Divine Nature of Christ and the human nature of Christ. The Virgin Mary does not intercede with the Most Holy Trinity to obtain grace for the human nature of Christ. Instead, she worships the whole Person of Jesus Christ, who is both human and Divine. If she were to pray for His human nature to receive grace, her prayer would be incompatible with her worship. Mary does not pray for Christ to be blessed by God or to be saved by God. Christ is Mary’s God and Savior, so He never requires her intercession in the least. Mary never interceded for Christ; from the time of the Annunciation, Mary always understood that Christ is God. Mary worships Christ; therefore, she cannot have a role in the mediation of graces received by His human nature. There can be no mediation, within the hypostatic union, between the Divine and human natures of Christ. For the two natures are united as One Person. Any claim to mediation between the two natures of Christ implies a separation which is incompatible with the dogma of the hypostatic union of two natures in One Person.” 
2. “The Virgin Mary is not the Mediatrix of the graces which flow from the Trinity, through Christ, to the Virgin Mary. When Mary receives grace, from Christ as the Source of grace by virtue of His Divinity, and through Christ as the Mediator of grace by virtue of His humanity, she is the recipient, not the Mediatrix, of that grace. She cannot be both Mediatrix and recipient of the same grace. She cannot stand as Mediatrix between Christ and herself.
“Suppose two persons have a disagreement. If they choose a mediator to attempt reconciliation between them, the mediator cannot be one of those two. A person cannot be judge over his own case. Neither can someone be a mediator of their own case. Thus, Mary cannot be Mediatrix in the case of the graces which she herself receives. She cannot be a Mediatrix in her own case.
“Mary cannot be the Mediatrix of the graces which Christ himself receives in His human nature, because Christ is not a Mediator in His own case. The graces given by the Divine Nature of Christ to the human nature of Christ have no mediator. Christ’s human nature is the Recipient of grace and Christ’s Divine Nature is the Source of that grace. Similarly, Mary cannot be both the recipient and the Mediatrix of the graces she herself receives. Christ is the Source and Mediator, and Mary is the recipient, of those graces. So the graces given from Christ to Mary have no Mediatrix, just as the graces given from the Divine Nature of Christ to the human nature of Christ have no Mediator. There is no Mediator between God and Christ, and so there is no Mediatrix between Christ and Mary.
“Christ is the Mediator of all grace with one exception. Christ is not the Mediator of grace given to Christ’s own human nature. Mary is the Mediatrix of all grace with two exceptions. Mary is not the Mediatrix of grace given to the human nature of Christ. Since Christ is not the Mediator of grace received by His human nature, neither can Mary be the Mediatrix of that grace. Mary is also not the Mediatrix of grace given to her from the Trinity through Christ; then she is the recipient, not the Mediatrix. Thus, in the singular case of the grace received by Mary, Christ is the Mediator without a Mediatrix. But, in all other cases where Christ is Mediator, Mary assists Christ as the Mediatrix.” 
Over at the blog, Canterbury Tales, Taylor Marshall asks ‘Is Mary the Mediatrix of ALL GRACES?‘
He makes a number of theological errors in his analysis. First, he entirely omits considering the graces Christ receives in His human nature, and the graces that Mary receives from Christ. Jesus Christ, in His human nature, and the Blessed Virgin Mary are the two most important recipients of grace. No one can establish that Mary is the Mediatrix of ALL graces without considering those two cases. Marshall omits any mention of these two cases. But, as shown above, Mary is not the Mediatrix in either case.
Second, he contradicts himself on the question as to whether Mary could be the Mediatrix of the graces mediated by Christ before her Immaculate Conception, or before Her Assumption. He says that she can be. But his description of the manner in which she mediates graces (we should say, the manner in which she assists Christ in His mediation of grace) portrays her as if she were entirely absent in the process, which is not really a type of mediation at all. A mediator or mediatrix who is absent cannot mediate.
“We also know that all the graces of the Old Testament were mediated in anticipation of Christ’s Incarnation and Death. Since Mary’s flesh and cooperation are necessary for the Incarnation and Death of Christ, these graces are also mediated with her role in mind. This is why Pope Pius IX says that the decree of Christ’s predestination is one and the same with that of Mary. So the graces of the Old Testament were mediated in light of her, though not actually dispensed by her.”
Marshall errs by portraying Christ’s role of mediation in Old Testament times as a mere anticipation of His future salvific acts. But Christ is God. And so, from the Cross in a certain Place, at a certain point within Time, the Eternal Son of God can mediate, dispense grace, redeem, save, justify, etc. without regard to Time or Place. The work of Christ during Old Testament times was His work directly, as God Incarnate, and not merely His work or the work of the Trinity in mere anticipation of the future Incarnation and His future Sacrifice on the Cross.
Similarly, when Mary entered Heaven at her Assumption (actually, she entered Heaven first at her death), she entered Eternity.
“The Virgin Mary is the Mediatrix of grace given even from the beginning of Time, before she was conceived on earth, because she dwells now with God, Who is Eternity, Who is beyond Time and Place. All the Elect in Heaven dwell with the Eternal Triune God. All the Elect in Heaven assist Christ in pouring out His grace upon all Creation, even from the very beginning of Time and Place.
“The Virgin Mary is the Mediatrix even of grace given by God before she was conceived. Once the perfect-Virgin Mary entered into Heaven, beyond Time and Place, she could then be present-tense to all persons, places, and events, everywhere and everywhen. From her place with God, Who is Eternity, the Virgin Mary fulfills her role as assistant to Christ as He pours forth grace throughout all Time and all Place, and beyond Time and Place.
“The merits of Christ’s Passion were applied from the Eternal Now to bring about the Immaculate Conception at a point in Time before Christ’s Passion had occurred. This is not a result of God, within Time, applying ‘foreseen’ merits. Rather, Christ’s Passion and death on the Cross is a source of grace, for all Creation, throughout all Time, from the Timelessness of Heaven. The actual present-tense merits of the Cross were applied, at the Immaculate Conception, from Christ, who is God, who is the Eternal Now.
“Heaven is outside of Time. Once the Virgin Mary entered into Heaven, she left Time and Place. From Heaven, with God Who is Eternity, the Virgin Mary can be present to all persons and all places throughout Time. In this way, God continually pours forth grace from Christ on the Cross, with the Virgin Mary as the holy Assistant and Mediatrix of Jesus Christ, to every Time and Place. Therefore, the Virgin Mary is the Mediatrix of graces given by Christ, even from the beginning of Time and Place. From her place in Eternity, the Virgin Mary is the Mediatrix of her Divine Son Jesus Christ, even for the multitude of graces given before she was conceived and born on Earth. Furthermore, anyone can ask God to give grace to persons throughout Time and Place, because God is beyond Time.” 
Taylor Marshall’s third error in his post is to present Mary’s role as Mediatrix as if it were the same type of role as Christ possesses, to be a mediator, but one that is subordinate to Christ. So her role would then be the same type of role, but still subordinate. He presents Mary as the New Eve, but he fails to understand that Eve was not merely subordinate to Adam; she had a different type of role. Eve was created to be a helper to Adam. Eve is a helper; Adam is not. Mary was created to be a helper to Christ. Mary is a helper; Christ is not. There is only one Mediator: Christ the Lord. Mary cannot be a sub-mediator. Her role is not as a second, but subordinate, Mediator. Mary and Jesus are not two mediators, both standing before God to mediate for humanity. Jesus, as God Incarnate, is the one Mediator. Mary has no role of mediation of her own. Her only role is to immerse herself entirely in Christ, in worship of Him as God, and assist Him in all that He does for our salvation.
The term Mediatrix is apt because it signifies an inherently feminine role, different in type from that of Christ. Men and women are given different roles in the plan of God for humanity, and so the Mediatrix is not merely a Mediator or sub-mediator who happens to be female. It is the inherently feminine role of helper to man, of helper to Christ as God-made-man.
“Mary’s role is neither the same as, nor similar to, Christ’s role. Mary has no role at all of her own, except to participate in Christ’s role. Mary selflessly immerses herself in Christ’s role, worshiping Him and serving Him.”
“Mary participates in Christ’s work of redemption in a way which is both radically subordinate to, and radically different from, Christ’s role.”
“As Mediatrix, Mary’s role is not to mediate, not even in a secondary or auxiliary way. Mary’s role as Mediatrix is to assist Christ as He alone mediates. Mary does not mediate, she merely assists the One who does mediate. In no way and in no sense of the word is Mary a mediator. The role of Mediatrix is merely to assist the one Mediator, Jesus Christ. Mary assists Christ in His work as Mediator, not by doing any mediating herself, not even ‘with and under’ Christ. Rather, she assists Christ by helping Him when He mediates. The Virgin Mary participates in Christ’s work of mediation, but she does not act as a mediator herself. Mary does not stand before God to Mediate for the People of God. Instead, she kneels before Christ in worship of Him, and she immerses herself in Christ selflessly, while He stands before the Father as the sole Mediator.
“How does Mary assist Christ in mediation? First and foremost, through the prayers and sufferings of her whole life. Mary offers her whole life and her whole self to God, as both a prayer and a sacrifice of the highest order, united to the supreme prayer and sacrifice of the life and death of Jesus Christ, for the sake of all those for whom Christ mediates.” 
    Quoted from my book, New Insights into the Deposit of Faith, from the chapter co-Redemptrix, Mediatrix, Advocatrix)
Ronald L. Conte Jr.
Roman Catholic theologian and Bible translator