But other people who are saved are saved without being kept from these things, right? Like all the other saints besides St. Mary? So why did that happen? For other exceptions to the way things generally work, like the right-hand thief being saved without baptism, there is some reason for it (there could not be baptism for him before he died) and some larger way that it fits into the way things work (he literally died with Christ, while we die with Christ and rise with him through our baptism). How does this Catholic doctrine about St. Mary fit in either of these ways?
Mary was preserved from all sin including the stain of original sin because it was fitting that God should dwell within a perfect ark for nine months. As the pope said in Ineffabilis Deus:
And indeed it was wholly fitting that so wonderful a mother should be ever resplendent with the glory of most sublime holiness and so completely free from all taint of original sin that she would triumph utterly over the ancient serpent. To her did the Father will to give his only-begotten Son – the Son whom, equal to the Father and begotten by him, the Father loves from his heart – and to give this Son in such a way that he would be the one and the same common Son of God the Father and of the Blessed Virgin Mary. It was she whom the Son himself chose to make his Mother and it was from her that the Holy Spirit willed and brought it about that he should be conceived and born from whom he himself proceeds.
Early Church Fathers on the Immaculate Conception
Justin Martyr
[Jesus] became man by the Virgin so that the course that was taken by disobedience in the beginning through the agency of the serpent might be also the very course by which it would be put down. Eve, a virgin and undefiled, conceived the word of the serpent and bore disobedience and death. But the Virgin Mary received faith and joy when the angel Gabriel announced to her the glad tidings that the Spirit of the Lord would come upon her and the power of the Most High would overshadow her, for which reason the Holy One being born of her is the Son of God. And she replied, “Be it done unto me according to your word” (Luke 1:38) (Dialogue with Trypho 100 [A.D. 155]).
St. Irenaeus (180 AD)
“Consequently, then, Mary the Virgin is found to be obedient, saying: ‘Behold, O Lord, your handmaid; be it done to me according to your word.’ Eve, however, was disobedient; and when yet a virgin, she did not obey… having become disobedient, was made the cause of death for herself and for the whole human race; so also Mary, betrothed to a man but nevertheless a virgin, being obedient, was made the cause of salvation for herself and for the whole human race… Thus, the knot of Eve’s disobedience was loosed by the obedience of Mary. What the virgin Eve had bound in unbelief, the virgin Mary loosed through faith.” (Against the Heresies, Book III [A.D. 180]).
St. Irenaeus uses “virginity” as a sign of sinlessness (i.e. Mary was sinless just as Eve was sinless before the Fall).
Hippolytus
He [Jesus] was the ark formed of incorruptible wood. For by this is signified that His tabernacle [Mary] was exempt from defilement and corruption (Orat. In Illud, Dominus pascit me, in Gallandi, Bibl. Patrum, II, 496 ante [A.D. 235]).
Origen
This Virgin Mother of the Only-begotten of God is called Mary, worthy of God, immaculate of the immaculate, one of the one (Homily 1 [A.D. 244]).
St. Ambrose of Milan (340-397)
Come, then, and search out Your sheep, not through Your servants or hired men, but do it Yourself. Lift me up bodily and in the flesh, which is fallen in Adam. Lift me up not from Sara but from Mary, a Virgin not only undefiled but a Virgin whom grace has made inviolate, free from every stain of sin (ut incorrupta sit virgo, sed virgo per gratium ab omni integra labe peccati)." (Commentary on Psalm 118, 22-30, 387 A.D.)
St. Ephraim the Syrian (ca. 350)
**You alone and your Mother are more beautiful than any others, for there is neither blemish in you nor any stains upon your Mother. Who of my children can compare in beauty to these? (Nisibene Hymns 27:8 [A. D. 361]).
“My Lady Most Holy, All-Pure, All-Immaculate, All-Stainless, All-Undefiled, All-Incorrupt, All-Inviolate …Spotless Robe of Him Who clothes Himself with light as with a garment …Flower unfading, purple woven by God, alone Most Immaculate.” (ibid.)**
Gregory Nazianzen
He was conceived by the virgin, who had been first purified by the Spirit in soul and body; for, as it was fitting that childbearing should receive its share of honor, so it was necessary that virginity should receive even greater honor (Sermon 38 [d. A.D. 390]).
St. Augustine (ca. AD 390)
Having excepted the Holy Virgin Mary, concerning whom, on account of the honor of the Lord, I wish to have absolutely no question when treating of sins,—for how do we know what abundance of grace for the total overcoming of sin was conferred upon her, who merited to conceive and bear Him in whom there was no sin?—so, I say again, with the exception of the Virgin, if we could have gathered together all those holy men and women, when they were living here, and had asked them whether they were without sin, what do we suppose would have been their answer? …if they had been so questioned, would they not have declared with a single voice: “If we say we have no sin, we deceive ourselves, and the truth is not in us!”? (Nature and Grace 36:42 [A.D. 415]).