The Catholic faith teaches that our Blessed Mother was conceived immaculate, that is, without original sin. This is something we must believe if we are Catholics. To celebrate this mystery, the Church begins by talking about original sin, and this is the point of the first reading. Original sin is a revealed truth, but it is a truth that can be known almost without Revelation. 
Anyone can see that something is wrong with the world. Why do we die? If we have an immortal soul, why don't we have an immortal body? Why do we so often prefer what is wrong to what is right? Isn't our nature good? Certainly, it is, and Revelation confirms that. God is good, and he created human nature good. In fact, we read in Genesis that after creating man, God saw everything that he had made, and behold, it was very good. 
So what is it that oppresses human nature, preventing it from expressing its natural goodness? Some blame society. If only man could grow up in a good environment, he would be good. It's true that bad example is harmful. Little children tend to be selfish and need to be taught to share. 
The problem exists from birth. Even without Revelation, we could come to the conclusion that something went wrong long ago, at the beginning of human history, or at least in some period that was very remote, because it afflicts all of mankind. 
The first reading explains that this is exactly what happened. It was the disobedience of the first man that caused this universal disorder. But how can his sin harm us? An analogy might help us. Consider the poor descendants of a man who was once very wealthy. If their ancestor had preserved his wealth, then his descendants would also be rich. But because he squandered it, they are poor. Adam is our ancestor, and God gave him riches of grace and other gifts, such as immortality, which we, as his descendants, should have inherited. But he squandered all these riches when he committed the first sin. Because of that original sin, we are born spiritually indigent. 
By God's grace, we are homeless. We cannot enter God's heavenly home, because he does not recognize us as his children, but as children of wrath. Our soul intends more easily toward evil than good, and our body intends inevitably toward death. Adam spiritually bankrupted himself with that sin, bankrupted himself and us, his descendants. God could allow this disaster to take place because he already had a remedy prepared. He had another source of grace and immortality. He wanted to make us heirs in Christ. He announced the remedy after the condemnation of our first parents, giving hope to them and mankind. In their hearing, he said to that ancient serpent who is the devil and Satan, I will put enmity between you and the woman and between your offspring and hers. He will strike at your head while you strike at his heel.
He was not talking about the woman who had just been defeated by the serpent and condemned by God, but another Woman. This other Woman was already known to the serpent, so that God would call her the Woman. She is the mother of the Incarnate Word, whom Christ calls woman at Cana and the cross. The early Church knew well of whom God was speaking in that passage. In the 2nd century AD, Saint Irenaeus wrote in reference to this passage, from then on he proclaimed that he who was to be born of a virgin after the likeness of Adam would be on the watch for the serpent's head. 
God or rather Christ is the New Adam who offers us grace and immortality, the grace and immortality that we were unable to obtain from the first Adam. Mary is the New Eve through whom we receive that inheritance. As the first Eve began her life without sin, so to the second Eve, Mary began her life without sin. She was conceived immaculate. 
Accordingly, blessed Pius IX writes, the fathers have never ceased to call the Father of God the lily without thorns, the land entirely intact, the virgin undefiled, immaculate, ever blessed and free from all contagion of sin. She from whom was formed the New Adam, the flawless, brightest and most beautiful paradise of innocence, immortality and delights, planted by God himself and protected against all the snares of the poisonous serpent, the incorruptible wood that the worm of sin had never corrupted, the fountain ever clear and sealed with the power of the Holy Spirit, the most holy temple, the treasure of immortality, the one and only daughter of life, not of death, the plant not of anger but of grace, the singular providence of God growing evergreen contrary to the common law, coming as it does from a corrupted and tainted root. 
How could an immaculate woman come from such a root? She was redeemed but in the most perfect way. We are redeemed after original sin has tainted us, as is the common law for Adam's descendants. Conceived as children of wrath and slaves of Satan, we are redeemed by baptism. 
Yet many parents do not baptize their children, leaving them at Satan's mercy until they grow up and decide for themselves to be baptized, if they ever do. Such parents are obviously making a serious mistake. Very different was the love of the Son of God for his mother. He did not want to leave her under Satan's power even for an instant. Therefore, in her case, redemption took the unique form of preservation from contracting original sin. 
For there are two ways to save someone. You can intervene after a fall to deal with the damage, or you can intervene before the fall to prevent it. The second is obviously more perfect. Our Lady was redeemed in this most perfect way to show that Christ is a perfect Redeemer. This is the brilliant insight of Blessed John Duns Scotus, who provided the foundation for the dogmatic definition. Redeemed in this way, our Blessed Mother is revealed as the New Eve, greeted by the angel as full of grace. Her Son will be great and will be called the Most High.
And if he is the Son of God, then he is God. He cannot have any stain of sin. Therefore, this man born of her is the New Adam. And if there is a New Adam and a New Eve, then God has refounded humanity. We are no longer doomed to live as children of wrath, because we can inherit grace from them. As no one inherits anything from Adam unless he is a child of Eve, so no one inherits anything from Christ unless he is a child of Mary. Eve was called the Mother of all the living, because all those who receive human life are her children.
But as Blessed Guericke said, she turned out to be more precisely the murderer of the living, or Mother of the dead, since the only fruit of her childbearing was death. The true Mother of all the living is the Immaculate, because the life she gives her children is eternal. Let us entrust ourselves to this dear Mother of all those who live in God. 
The second reading reminds us that we are called to become like our Heavenly Mother. But this isn't so clear in the translation we heard. It says, God chose us to be holy and without blemish before him. It would be better to translate those words as holy and immaculate before him. This makes it clear why the Church chose that reading. Mary was conceived holy and immaculate. We are chosen to become holy and immaculate. What we were not when we began our life on earth, we must become to begin our life in heaven. 
Mary Immaculate is our model, and so let us imitate her virtues, especially her faith, charity, and obedience. She not only shows what grace can do as our Mediatrix, she even obtains the grace for us to become like her. But we must choose to follow her rather than the serpent, however much it may tempt us. There must be enmity, total opposition between us and the serpent, between us and sin. To be holy and immaculate is to be totally opposed to sin. So let us live evermore fully the life of grace, that grace which crushes the head of the ancient serpent.
Let us strive to be holy and immaculate before God in charity, like our Blessed Mother.
Praised be Jesus and Mary … now and forever.