Objection 1. It would seem that by being sanctified in the womb the Blessed Virgin was not preserved from all actual sin. For, as we have already stated (3), after her first sanctification the fomes remained in the Virgin. Now the motion of the fomes, even if it precede the act of the reason, is a venial sin, albeit extremely slight, as Augustine says in his work De Trinitate [Cf. Sent. ii, D, 24]. Therefore there was some venial sin in the Blessed Virgin.
Objection 2. Further, Augustine (Qq. Nov. et Vet. Test. lxxiii on Luke 2:35: "Thy own soul a sword shall pierce") says that the Blessed Virgin "was troubled with wondering doubt at the death of our Lord." But doubt in matters of faith is a sin. Therefore the Blessed Virgin was not preserved from all actual sin.
Objection 3. Further, Chrysostom (Hom. xlv in Matth.) expounding the text: "Behold thy mother and thy brethren stand without, seeking thee," says: "It is clear that they did this from mere vain glory." Again, on John 2:3: "They have no wine," the same Chrysostom says that "she wished to do them a favor, and raise herself in their esteem, by means of her Son: and perchance she succumbed to human frailty, just as did His brethren when they said: 'Manifest Thyself to the world.'" And a little further on he says: "For as yet she did not believe in Him as she ought." Now it is quite clear that all this was sinful. Therefore the Blessed Virgin was not preserved from all sin.
On the contrary, Augustine says (De Nat. et Grat. xxxvi): "In the matter of sin, it is my wish to exclude absolutely all questions concerning the holy Virgin Mary, on account of the honor due to Christ. For since she conceived and brought forth Him who most certainly was guilty of no sin, we know that an abundance of grace was given her that she might be in every way the conqueror of sin."
I answer that, God so prepares and endows those, whom He chooses for some particular office, that they are rendered capable of fulfilling it, according to 2 Corinthians 3:6: "(Who) hath made us fit ministers of the New Testament." Now the Blessed Virgin was chosen by God to be His Mother. Therefore there can be no doubt that God, by His grace, made her worthy of that office, according to the words spoken to her by the angel (Luke 1:30-31): "Thou hast found grace with God: behold thou shalt conceive," etc. But she would not have been worthy to be the Mother of God, if she had ever sinned. First, because the honor of the parents reflects on the child, according to Proverbs 17:6: "The glory of children are their fathers": and consequently, on the other hand, the Mother's shame would have reflected on her Son. Secondly, because of the singular affinity between her and Christ, who took flesh from her: and it is written (2 Corinthians 6:15): "What concord hath Christ with Belial?" Thirdly, because of the singular manner in which the Son of God, who is the "Divine Wisdom" (1 Corinthians 1:24) dwelt in her, not only in her soul but in her womb. And it is written (Wisdom 1:4): "Wisdom will not enter into a malicious soul, nor dwell in a body subject to sins."
We must therefore confess simply that the Blessed Virgin committed no actual sin, neither mortal nor venial; so that what is written (Canticles 4:7) is fulfilled: "Thou art all fair, O my love, and there is not a spot in thee," etc.
Reply to Objection 1. After her sanctification the fomes remained in the Blessed Virgin, but fettered; lest she should be surprised by some sudden inordinate act, antecedent to the act of reason. And although the grace of her sanctification contributed to this effect, yet it did not suffice; for otherwise the result of her sanctification would have been to render impossible in her any sensual movement not preceded by an act of reason, and thus she would. not have had the fomes, which is contrary to what we have said above (Article 3). We must therefore say that the above mentioned fettering (of the fomes) was perfected by divine providence not permitting any inordinate motion to result from the fomes.
Reply to Objection 2. Origen (Hom. xvii in Luc.) and certain other doctors expound these words of Simeon as referring to the sorrow which she suffered at the time of our Lord's Passion. Ambrose (in Luc. 2:35) says that the sword signifies "Mary's prudence which took note of the heavenly mystery. For the word of God is living and effectual, and more piercing than any two-edged sword" (Hebrews 4:12).
Others again take the sword to signify doubt. But this is to be understood of the doubt, not of unbelief, but of wonder and discussion. Thus Basil says (Ep. ad Optim.) that "the Blessed Virgin while standing by the cross, and observing every detail, after the message of Gabriel, and the ineffable knowledge of the Divine Conception, after that wondrous manifestation of miracles, was troubled in mind": that is to say, on the one side seeing Him suffer such humiliation, and on the other considering His marvelous works.
Reply to Objection 3. In those words Chrysostom goes too far. They may, however, be explained as meaning that our Lord corrected in her, not the inordinate motion of vain glory in regard to herself, but that which might be in the thoughts of others.
Source: Summa Theologica (New Advent)