Some heretics, like Luther and Calvin, have wanted to try to find some sin of the Virgin Mary in the Gospels. In that way, the Mother of Christ would have been negligent towards the Child Jesus in the Temple and she would have lost patience with Him after having found Him.
The contrary must be said, that the absolute perpetual enmity between the Blessed Virgin and Satan expressed in Genesis 3:15 implicitly affirms her immunity from sin, at least mortal.
What’s more, the Archangel Gabriel’s salutation, “full of grace” is crucial. Mortal sin is excluded because this fullness contains the confirmation in grace. And as for venial sin, as St. Albert the Great said in his Mariale: “Where there is venial sin, there is a certain lack of grace; but Mary was full of grace; therefore there was no sin in her.”
Mention must also be made to this text from the Canticle of Canticles that the holy Church applies to the Most Holy Virgin, “Thou art all fair, O my love, and there is not a spot in thee!”
The Fathers of the Church echo the sacred words. St. Augustine gave a theological motive: “In order to honor the Lord, I want there to be no question [about Mary], with respect to sin.”
St. Bernard also affirms her immunity from actual sin: “And I think for my part that a most abundant sanctifying blessing descended on her, which not only sanctified her birth but subsequently guarded her life untouched by all sin, which, as it is believed, was certainly not given to any other among those who were born of a woman. It was suitable indeed for the Queen of Virgins, through the privilege of a singular sanctity, to lead a live without any sin, she who gave birth to the Redeemer from sin and death and obtained for all of us the gift of life and justice.”
The Magisterium’s voice sang out the same truth. In this way the Council of Trent affirms that a man “is [not] able, during his whole life, to avoid all sins, even those that are venial, except by a special privilege from God, as the Church holds in regard of the Blessed Virgin.” (Sixth Session, Canon 23)
Pope Pius IX, in the Bull Ineffabilis Deus, which proclaims the Immaculate Conception of the Blessed Virgin, affirms moreover: “Therefore, far above all the angels and all the saints so wondrously did God endow her with the abundance of all heavenly gifts poured from the treasury of His divinity that this mother, ever absolutely free of all stain of sin, all fair and perfect, would possess that fullness of holy innocence and sanctity than which, under God, one cannot even imagine anything greater, and which, outside of God, no mind can succeed in comprehending fully.”
Theology only repeats this tradition by affirming that the Virgin committed no actual sin. This perfection is very suitable for several reasons, as St. Thomas Aquinas says:
The shame of the parents reflects on the child. The shame of sin would reflect on the Son of God.
The singular affinity between Mary and the Word forbids it: “What concord hath Christ with Belial?”
Finally, the angelic doctor cites the Book of Wisdom: “Wisdom [here, the Word] will not enter into a malicious soul, nor dwell in a body subject to sins.” (Summa, III, q.27, a.4)
Let us leave the last word to St. Ephrem who speaks to us about the Virgin, “without stain, without blemish, without corruption, but entirely modest, completely foreign to any impurity, to any defect, the spouse of God, our mistress.”
