Objection 1. It would seem that the Blessed Virgin was sanctified before animation. Because, as we have stated (1), more grace was bestowed on the Virgin Mother of God than on any saint. Now it seems to have been granted to some, to be sanctified before animation. For it is written (Jeremiah 1:5): "Before I formed thee in the bowels of thy mother, I knew thee": and the soul is not infused before the formation of the body. Likewise Ambrose says of John the Baptist (Comment. in Luc. i, 15): "As yet the spirit of life was not in him and already he possessed the Spirit of grace." Much more therefore could the Blessed Virgin be sanctified before animation.
Objection 2. Further, as Anselm says (De Concep. Virg. xviii), "it was fitting that this Virgin should shine with such a purity that under God none greater can be imagined": wherefore it is written (Canticles 4:7): "Thou art all fair, O my love, and there is not a spot in thee." But the purity of the Blessed Virgin would have been greater, if she had never been stained by the contagion of original sin. Therefore it was granted to her to be sanctified before her flesh was animated.
Objection 3. Further, as it has been stated above, no feast is celebrated except of some saint. But some keep the feast of the Conception of the Blessed Virgin. Therefore it seems that in her very Conception she was holy; and hence that she was sanctified before animation.
Objection 4. Further, the Apostle says (Romans 11:16): "If the root be holy, so are the branches." Now the root of the children is their parents. Therefore the Blessed Virgin could be sanctified even in her parents, before animation.
On the contrary, The things of the Old Testament were figures of the New, according to 1 Corinthians 10:11: "All things happened to them in figure." Now the sanctification of the tabernacle, of which it is written (Psalm 45:5): "The most High hath sanctified His own tabernacle," seems to signify the sanctification of the Mother of God, who is called "God's Tabernacle," according to Psalm 18:6: "He hath set His tabernacle in the sun." But of the tabernacle it is written (Exodus 40:31-32): "After all things were perfected, the cloud covered the tabernacle of the testimony, and the glory of the Lord filled it." Therefore also the Blessed Virgin was not sanctified until after all in her was perfected, viz. her body and soul.
I answer that, The sanctification of the Blessed Virgin cannot be understood as having taken place before animation, for two reasons. First, because the sanctification of which we are speaking, is nothing but the cleansing from original sin: for sanctification is a "perfect cleansing," as Dionysius says (Div. Nom. xii). Now sin cannot be taken away except by grace, the subject of which is the rational creature alone. Therefore before the infusion of the rational soul, the Blessed Virgin was not sanctified.
Secondly, because, since the rational creature alone can be the subject of sin; before the infusion of the rational soul, the offspring conceived is not liable to sin. And thus, in whatever manner the Blessed Virgin would have been sanctified before animation, she could never have incurred the stain of original sin: and thus she would not have needed redemption and salvation which is by Christ, of whom it is written (Matthew 1:21): "He shall save His people from their sins." But this is unfitting, through implying that Christ is not the "Saviour of all men," as He is called (1 Timothy 4:10). It remains, therefore, that the Blessed Virgin was sanctified after animation.
Reply to Objection 1. The Lord says that He "knew" Jeremias before he was formed in the womb, by knowledge, that is to say, of predestination: but He says that He "sanctified" him, not before formation, but before he "came forth out of the womb," etc. As to what Ambrose says, viz. that in John the Baptist there was not the spirit of life when there was already the Spirit of grace, by spirit of life we are not to understand the life-giving soul, but the air which we breathe out [respiratus]. Or it may be said that in him as yet there was not the spirit of life, that is the soul, as to its manifest and complete operations.
Reply to Objection 2. If the soul of the Blessed Virgin had never incurred the stain of original sin, this would be derogatory to the dignity of Christ, by reason of His being the universal Saviour of all. Consequently after Christ, who, as the universal Saviour of all, needed not to be saved, the purity of the Blessed Virgin holds the highest place. For Christ did not contract original sin in any way whatever, but was holy in His very Conception, according to Luke 1:35: "The Holy which shall be born of thee, shall be called the Son of God." But the Blessed Virgin did indeed contract original sin, but was cleansed therefrom before her birth from the womb. This is what is signified (Job 3:9) where it is written of the night of original sin: "Let it expect light," i.e. Christ, "and not see it"--(because "no defiled thing cometh into her," as is written in Wisdom 7:25), "nor the rising of the dawning of the day," that is of the Blessed Virgin, who in her birth was immune from original sin.
Reply to Objection 3. Although the Church of Rome does not celebrate the Conception of the Blessed Virgin, yet it tolerates the custom of certain churches that do keep that feast, wherefore this is not to be entirely reprobated. Nevertheless the celebration of this feast does not give us to understand that she was holy in her conception. But since it is not known when she was sanctified, the feast of her Sanctification, rather than the feast of her Conception, is kept on the day of her conception.
Reply to Objection 4. Sanctification is twofold. One is that of the whole nature: inasmuch as the whole human nature is freed from all corruption of sin and punishment. This will take place at the resurrection. The other is personal sanctification. This is not transmitted to the children begotten of the flesh: because it does not regard the flesh but the mind. Consequently, though the parents of the Blessed Virgin were cleansed from original sin, nevertheless she contracted original sin, since she was conceived by way of fleshly concupiscence and the intercourse of man and woman: for Augustine says (De Nup. et Concup. i): "All flesh born of carnal intercourse is sinful."
Source: Summa Theologica (New Advent)