This feast is one of my favorites. It forms one leg of that triad of feasts which, by highlighting aspects of God's favor to Mary at the beginning, middle, and end of her earthly life, shows us what are all called to be. Her status as Mother of God is the paradigm of how the disciples of her divine Son are to bear him into the world by surrendering their hearts to God; the Assumption is a proleptic anticipation and figura of what all the definitively saved shall be. But from the start, she was prepared as a fitting vessel for her Son by being kept free of all "stain of original sin," to use the words of the dogma's definition. What baptism does for each of us at some point in our lives, God did for her directly from the very beginning. Mariology shows us, in perfect form, the vocation of the Christian. By thus signifying, Mary the Mother of the Church helps to bring about what she signifies. Like the Church herself, she is a sacrament.
Like all other distinctively Catholic doctrines, the doctrine of the Immaculate Conception (DIM) is widely misunderstood, largely because of that "stain" idea and its history. Before I move on to the dogma's practical import, I want to clear up what I see as the major misunderstanding.
The Council of Trent defined:
If anyone says that the transgression of Adam injured him alone and not his posterity, and that the holiness and justice which he received from God, which he lost, he lost for himself alone and not for us also; or that he, being defiled by the sin of disobedience, has transfused only death and the pains of the body into the whole human race, but not sin also, which is the death of the soul: let him be anathema. For he contradicts the Apostle who says: "By one man sin entered into the world and by sin death; and so death passed upon all men, in whom all have sinned" (Romans 5:12).
St. Augustine interpreted that passage from Romans as implying that the "sin" which the first man introduced into the world involves the personal guilt of all who are descended from him. That interpretation has been very influential in Western theology. Yet gradually and inexorably, the Catholic Church has repudiated it. Thus CCC §405:
Although it is proper to each individual, original sin does not have the character of a personal fault in any of Adam's descendants. It is a deprivation of original holiness and justice, but human nature has not been totally corrupted: it is wounded in the natural powers proper to it, subject to ignorance, suffering and the dominion of death, and inclined to sin—an inclination to evil that is called "concupiscence".
Therefore, original sin is distinct from the first sin as its consequence on the one hand, and from concupiscence on the other hand, which is one of its chief consequences. It is a state of alienation from God that we all inherit from our first parents. Mary's preservation from the "stain" of original sin consists primarily in her never having existed in that state. She was always kecharitomene, "full of grace," and grew ever fuller with every choice she made. We are called to do the same.
But how can we identify with Mary? Isn't she "too perfect"? She is and always will be the greatest of the "saints," God's holy ones. None of us will ever be as great as she in the Kingdom of God. But that is not primarily her doing. Sure, she cooperated with the Holy Spirit with her fiat and all her other choices, as we are expected to. But her special privileges were neither earned nor able to be earned. That should be a great comfort to us. Even as we stumble and struggle in our spiritual combat, we can know that God will fill up what is lacking in those who persevere—just as our own suffering can and should "fill what is lacking" (Colossians 1:24) in the divine suffering that made our salvation possible. By the Mother of God's intercession, that can be more than enough.