Was the Blessed Virgin conceived in sin? The answer is no, for as Augustine writes: “When sin is treated, there can be no inclusion of Mary in the discussion.” And Anselm says: “It was fitting that the Virgin should be resplendent with a purity greater than which none under God can be conceived.” Purity here is to be taken in the sense of pure innocence under God, such as was in Christ.
The contrary, however, is commonly asserted on two grounds. First, the dignity of Her Son, who, as universal Redeemer, opened the gates of heaven. But if blessed Mary had not contracted original sin, She would not have needed the Redeemer, nor would He have opened the door for Her because it was never closed. For it is only closed because of sin, above all original sin.
In respect to this first ground, one can argue from the dignity of Her Son qua Redeemer, Reconciler, and Mediator, that She did not contract original sin.
For a most perfect mediator exercises the most perfect mediation possible in regard to some person for whom he mediates. Thus Christ exercised a most perfect act of mediation in regard to some person for whom He was Mediator. In regard to no person did He have a more exalted relationship than to Mary. Such, however, would not have been true had He not preserved Her from original sin.
The proof is threefold: in terms of God to whom He reconciles; in terms of the evil from which He frees; and in terms of the indebtedness of the person whom He reconciles.
First, no one absolutely and perfectly placates anyone about to be offended in any way unless he can avert the offense. For to placate only in view of remitting the offense once committed is not to placate most perfectly. But God does not undergo offense because of some experience in Himself, but only because of sin in the soul of a creature. Hence, Christ does not placate the Trinity most perfectly for the sin to be contracted by the sons of Adam if He does not prevent the Trinity from being offended in someone, and if the soul of some child of Adam does not contract such a sin; and thus it is possible that a child of Adam not have such a sin.
Secondly, a most perfect mediator merits the removal of all punishment from the one whom he reconciles. Original sin, however, is a greater privation than the lack of the vision of God. Hence, if Christ most perfectly reconciles us to God, He merited that this most heavy of punishments be removed from some one person. This would have been His Mother.
Further, Christ is primarily our Redeemer and Reconciler from original sin rather than actual sin, for the need of the Incarnation and suffering of Christ is commonly ascribed to original sin. But He is also commonly assumed to be the perfect Mediator of at least one person, namely, Mary, whom He preserved from actual sin. Logically one should assume that He preserved Her from original sin as well.
Thirdly, a person reconciled is not absolutely indebted to his mediator, unless he receives from that mediator the greatest possible good. But this innocence, namely, preservation from the contracted sin or from the sin to be contracted, is available from the Mediator. Thus, no one would be absolutely indebted to Christ as Mediator unless preserved from original sin. It is a greater good to be preserved from evil than to fall into it and afterwards be freed from it. If Christ merited grace and glory for so many souls, who, for these gifts, are indebted to Christ as Mediator, why should no soul be His debtor for the gift of its innocence? And why, since the blessed Angels are innocent, should there be no human soul in heaven (except the human soul of Christ) who is innocent, that is, never in the state of original sin?