Today is the Solemnity of the Immaculate Conception of the Blessed Virgin Mary. The Church rightly teaches that Mary was conceived without the stain of Original Sin, based upon the anticipated merits of the passion, death, and resurrection of her Son, Jesus Christ.

In 1854, Pope Pius XI proclaimed in Ineffabilis Deus the following:

We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.

This makes sense on many different levels.

• God exists outside of time and is not restricted by the chronological sequence of events that occur within time. 
• The Holy Spirit could not have ‘overshadowed’ Mary to form Jesus in her womb if she was in the state of Original Sin. Mary’s union with the Holy Spirit is a spousal union. “What God had joined, let none rend asunder.” This is an important point that helped me finally understand more solidly the whole ‘Immaculate Conception’ thing. Her spousal union with the Holy Spirit and the Spirit’s ‘overshadowing’ Mary required her to be sinless. Not just preserved from Original Sin, but also the stain it leaves behind (concupiscence.). Sin blocks grace from the soul; mortal sin is deadly and separates us from God, while venial sin distances us from God. Mary’s spousal union with the Spirit would have been ruptured if she was capable of sinning. Remember: “What God had joined, let none rend asunder.”  If she was in any state of sin, the Holy Spirit could not have joined with her in the first place. 
• Therefore, how can the Holy Spirit’s spousal union with Mary be maintained at all if she had concupiscence? It couldn’t. Therefore, Mary could not have Original Sin, and by not being subjected to it or having its stain on her soul, she was incapable of committing venial and mortal sins. This is where all other humans differ from Mary. Although by Baptism we’ve had Original Sin removed, its stain remains, and by this concupiscence, we sin. With Mary, since the stain was removed concupiscence was never a part of her being. But while Christians have received the Holy Spirit in Baptism, our union with the Spirit is not to the same degree as Mary’s. Ours is not a spousal union; sin can rupture it. Hence we need the Sacraments to repair the rupture.
• Since she bore Him in her womb for nine months, she could not even commit venial and mortal sins during this period as this would place Jesus under the domain of Satan, since a fetus is physically a part of the mother. (While not culpable for the mother’s sins, nor capable of sinning itself, a fetus would still be affected by them.) Her sinless behavior obviously would have continued after Jesus’ birth. This is the basis for the teachings of Sts. Lous de Montfort and Maximilian Kolbe when they wrote that Mary’s will was always in conformity with God’s will. Kolbe especially emphasized this. 
• So, the Dogma of the Immaculate Conception has the effect also of rendering the rubrics of administering the sacraments more meaningful, given the role of the Holy Spirit in everything. The Spirit joined with Mary because she was without sin. We are baptized and Original Sin is removed and then the Holy Spirit enters our soul and later we can receive the remaining sacraments. 
• Some critics point out that St. Paul said somewhere in his letters that “all have sinned.” Well, this cannot possibly mean ‘all’ as in ‘everyone;’ for would this ‘all’ include Jesus? I think Paul meant  ‘all born of women’ in the normal manner of birthing. If someone is still going to make the point that ‘all’ inherit Original Sin, and then Mary would still need redeeming, then we go back to the original declaration of the dogma of her immaculate conception that she was redeemed by the anticipated merits of Jesus Christ and so was prevented from having the stain of Original Sin in the first place. (please refer to the first bullet for any chronological objections.

  I brought up this point in an older post:

• “…wouldn’t God, Who knew from all Eternity His plan of Salvation, and decided that His Son would be born of a woman rather than Incarnate as a mighty king and lord fully grown, wouldn’t He have taken great pains to decide upon the formation of she who would bear His Son? If YOU had the opportunity to design your own mother, wouldn’t YOU insist that she the among the most beautiful, intelligent, and talented of all? One of the Ten Commandments holds that we should “Honor our Father and Mother,” well, wouldn’t God also follow that? Even one was to declare that He is not subject to His own Commandments and laws, why wouldn’t He follow that one at least, to provide an example?” An addendum to this point is that if YOU could make your own mother, and could also make her perfectly pure and holy, wouldn’t you?

  In an even older post I said:

• “…One could argue then that why couldn’t Jesus have been conceived immaculately? He could have, but the difficulty in that would be that He still would be in Mary’s womb, and what would be the barrier between Him and Original Sin? His own sacrifice on the cross, decades later? He is divine and sinless, so His own death was not for Himself, He died for humanity. So Mary, by sharing her body and blood with Jesus in her womb, would benefit from the eventual sacrifice of Jesus. Mary is the physical barrier between Jesus and her ancestral line, caught in Original Sin like the rest of humanity. The physical barrier protecting Mary from her mother’s state of Original Sin was Jesus, operating from the fullness of time, as God dwells in Eternity.”

So, there it is! See how it all connects? Remove Original Sin and the free operation of the Holy Spirit can begin in souls. With Mary, it required her to be preserved from all sin so the Holy Spirit could join her in an eternal spousal union so that Jesus could be formed in her womb (and later so that Mary could participate in the distribution of graces from the Holy Spirit; but that’s a whole ’nother topic.) With us, it required us to be Baptized so the Holy Spirit could join us in a sacramental union so that we could be formed into the Mystical Body of Christ (and receive the graces from which the Holy Spirit is the source. Whoops, ’nother topic!) 

My two other books are still available! "The Stations of the Cross for Alcoholics" and "The Recovery Rosary: Reflections for Alcoholics and Addicts" (Thank you!!)