How Could Someone Be Born Without Original Sin?:
Adam and Eve, by disobeying God's command not to eat the fruit of the Tree of the Knowledge of Good and Evil (Genesis 2:16-17; Genesis 3:1-19), brought sin and death into this world. Roman Catholic doctrine and tradition hold that Adam's sin has been passed down from generation to generation. It is not simply that the world around us has been corrupted by Adam's sin in such a way that all those who have followed have found it nearly impossible not to sin (an admittedly simplified version of the Eastern Christian view), but that our very nature was corrupted in such a way that life without sin is impossible. This corruption of our nature, passed down from father to child, is what we call Original Sin.
Roman Catholic doctrine and tradition, however, also hold that three people were born without Original Sin. Yet how can that be, if Original Sin is physically passed from generation to generation? The answer is different in each case.
Jesus Christ: Conceived Without Sin:
Jesus Christ was born without Original Sin because He was conceived without Original Sin. The Son of the Blessed Virgin Mary, Christ is also the Son of God. In the Roman Catholic tradition, Original Sin is, as I mentioned, passed down from father to child; the transmission occurs through the sexual act. Since Christ's Father is God Himself, there was no Original Sin to be passed down. Conceived by the Holy Spirit through Mary's willing cooperation at the Annunciation, Christ was not subject to Adam's sin or its effects.
The Blessed Virgin Mary: Conceived Without Sin:
The Blessed Virgin Mary, too, was born without Original Sin because she was conceived without Original Sin. We call her preservation from Original Sin her Immaculate Conception.
Mary, however, was preserved from Original Sin in a different way from Christ. She is the daughter of God in the same way that Christ is the Son of God. Mary's father, Saint Joachim, was a man, and like all men descended from Adam, he was subject to Original Sin. Under normal circumstances, Joachim would have passed that sin on to Mary through her conception in the womb of Saint Anne.
God, however, had other plans. Saint Mary, in the words of Pope Pius IX, preserved from Original Sin "in the first instance of her conception, by a singular grace and privilege granted by Almighty God." (See the Apostolic Constitution Ineffabilis Deus, in which Pius IX infallibly proclaims the doctrine of Mary's Immaculate Conception.) That "singular grace and privilege" was granted to Mary because of God's foreknowledge that she would, at the Annunciation, consent to be the mother of His Son. Mary had free will; she could have said no; but God knew that she would not. And so, "in view of the merits of Jesus Christ, the Savior of the human race," God preserved Mary from the stain of Original Sin that had been mankind's condition since the Fall of Adam and Eve.
It's important to note that Mary's preservation from Original Sin was not necessary; God did it of His great love for her, and through the merits of Christ's redemptive action. Since Original Sin is passed down from father to child, Christ would have been conceived without Original Sin even if Mary had been born with Original Sin. Thus, the common Protestant objection that Mary's Immaculate Conception would necessarily require an immaculate conception of her parents, and of theirs, all the way back to Adam is based on a misunderstanding of how Original Sin is transmitted.
For Christ to be born without Original Sin, it was not necessary for Mary to be born without Original Sin. God's preservation of Mary from Original Sin was a pure act of love. Mary was redeemed by Christ; but her redemption was accomplished by God at the moment of her conception, in anticipation of the redemption of man that Christ would work through His Death upon the Cross. (For a more detailed discussion of Mary's Immaculate Conception, see What Is the Immaculate Conception? and the profile of the Feast of the Immaculate Conception.)
John the Baptist: Born Without Original Sin:
Many Catholics today are surprised to learn that Catholic tradition holds that a third person was born without Original Sin. There is a difference, however, between Saint John the Baptist's birth without Original Sin and that of Christ and Mary: Unlike Jesus and the Blessed Virgin, John the Baptist was conceived with Original Sin, yet he was born without it. How could that be?
John's father, Zachary (or Zacharias), was, like Mary's father, Joachim, subject to Original Sin. But God did not preserve John the Baptist from the stain of Original Sin at his conception. So John, like all of us descended from Adam, was himself subject to Original Sin. But then a wondrous event occurred. Mary, having been told by the Angel Gabriel at the Annunciation that her cousin Elizabeth, the mother of John the Baptist, was pregnant in her old age (Luke 1:36-37), went to help her cousin (Luke 1:39-40).
The Visitation, as this act of charity is known, is found in Luke 1:39-56. It is a touching scene of love of two cousins for each other, but it also tells much about the spiritual state of Mary and of John the Baptist. The Angel Gabriel had declared Mary "blessed among women" at the Annunciation (Luke 1:28), and Elizabeth, filled with the Holy Spirit, repeats his greeting and amplifies it: "Blessed art thou among women, and blessed is the fruit of thy womb" (Luke 1:42).
And while the cousins are greeting each other, "the infant [John the Baptist] leaped in her [Elizabeth's] womb" (Luke 1:41). That "leap" has traditionally been seen as John's acknowledgment of the presence of Christ; in the womb of his mother, who was filled with the Holy Spirit, he too was filled with the Spirit, and his "leap" represents a type of Baptism. As the Catholic Encyclopedia notes in its entry on St. John the Baptist:
Now during the sixth month, the Annunciation had taken place, and, as Mary had heard from the angel the fact of her cousin's conceiving, she went "with haste" to congratulate her. "And it came to pass, that when Elizabeth heard the salutation of Mary, the infant"—filled, like the mother, with the Holy Ghost—"leaped for joy in her womb", as if to acknowledge the presence of his Lord. Then was accomplished the prophetic utterance of the angel that the child should "be filled with the Holy Ghost even from his mother's womb." Now as the presence of any sin whatever is incompatible with the indwelling of the Holy Ghost in the soul, it follows that at this moment John was cleansed from the stain of original sin.
So John, unlike Christ and Mary, was conceived with Original Sin; but three months before his birth, he was cleansed of Original Sin, and thus was born without Original Sin.
Being Born Without Original Sin Versus Being Conceived Without Sin:
As we have seen, the circumstances through which each of the three people—Jesus Christ, the Blessed Virgin Mary, and Saint John the Baptist—were born without Original Sin were different from one another; but the effects, too, are different, at least for John the Baptist. Christ and Mary, having never been subject to Original Sin, were never exposed to the corrupting effects of Original Sin, which remain after Original Sin is forgiven. Those effects include a weakening of our will, a clouding of our intellect, and concupiscence—the tendency to indulge our desires rather than to subordinate them to the right operation of our reason. Those effects are why we still fall prey to sin even after our baptism, and the absence of those effects is why Christ and Mary could remain free from sin throughout their lives.
John the Baptist, however, was subject to Original Sin, even though he was cleansed of it before his birth. That cleansing placed him in the same position that we find ourselves after our baptism: freed from Original Sin, but still subject to its effects. Thus Catholic doctrine does not hold that John the Baptist remained free from sin throughout his life; indeed, the likelihood that he did so was quite remote. The special circumstances of his cleansing from Original Sin notwithstanding, John the Baptist remained, as we do, under the shadow of sin and death that Original Sin casts upon man.