The subject of this immunity from original sin is the person of Mary at the moment of the creation of her soul and its infusion into her body.
The term conception does not mean the active or generative conception by her parents. Her body was formed in the womb of the mother, and the father had the usual share in its formation. The question does not concern the immaculateness of the generative activity of her parents. Neither does it concern the passive conception absolutely and simply (conceptio seminis carnis, inchoata), which, according to the order of nature, precedes the infusion of the rational soul. The person is truly conceived when the soul is created and infused into the body. Mary was preserved exempt from all stain of original sin at the first moment of her animation, and sanctifying grace was given to her before sin could have taken effect in her soul.
The formal active essence of original sin was not removed from her soul, as it is removed from others by baptism; it was excluded, it never was in her soul. Simultaneously with the exclusion of sin. The state of original sanctity, innocence, and justice, as opposed to original sin, was conferred upon her, by which gift every stain and fault, all depraved emotions, passions, and debilities, essentially pertaining to original sin, were excluded.
The immunity from original sin was given to Mary by a singular exemption from a universal law through the same merits of Christ, by which other men are cleansed from sin by baptism. Mary needed the redeeming Saviour to obtain this exemption, and to be delivered from the universal necessity and debt (debitum) of being subject to original sin. The person of Mary, in consequence of her origin from Adam, should have been subject to sin, but, being the new Eve who was to be the mother of the new Adam, she was, by the eternal counsel of God and by the merits of Christ, withdrawn from the general law of original sin. Her redemption was the very masterpiece of Christ’s redeeming wisdom. He is a greater redeemer who pays the debt that it may not be incurred than he who pays after it has fallen on the debtor. (from Catholic Encyclopedia, New Advent).
Let us entreat the nine choirs of Angels and the different orders of Saints to permit us celebrate with them the privilege of Mary.
And then, with joy and gratitude, while we confess our unworthiness, let us sing “Tota pulchra.” “You are all fair, O Mary and the stain of original sin is not in you. You are the glory of Jerusalem, you are the joy of Israel, you are the honor of our people, O Mary, O Mary most prudent, Mother most clement, pray for us, intercede for us with Our Lord Jesus Christ.”
Father, you prepared the Virgin Mary to be the worthy mother of your Son. You let her share beforehand in the salvation Christ would bring by his death, and kept her sinless from the first moment of her conception. Help us by her prayers to live in your presence without sin. We ask this through Christ our Lord. Amen.