“I will put enmities between thee and the woman, and thy seed and her seed: she shall crush thy head, and thou shalt lie in wait for her heel.”
To understand the Immaculate Conception, we must go back to this verse in Genesis chapter three, placed immediately after the fall of Adam and Eve. If the word “enmities” sounds archaic, we can translate it into modern English by saying that God announced His intention to make a woman who was the “polar opposite” of the devil. She would respond to the devil's evil with absolute good; to his sinfulness with perfect innocence; to his hatred of God and man with abundant love for both. She would literally be the Mother of Truth, completely opposed to the Father of Lies. In order to achieve this juxtaposition, she would be perfectly sinless. This perfection would, of course, be lifelong—but for it to be God's perfect work, her sinlessness would begin with the first moment of her existence—which is to say that this woman, whom we know as the Blessed Virgin Mary was perfectly sinless from the moment of her conception in the womb of her mother, Anna, the wife of Joachim.
The Immaculate Conception refers to Mary’s origin, not that of Jesus—but it must be noted that this perfect sinlessness was necessary if the Second Person of God were to receive His sinless human body from Mary. Remember that every atom in the body of the baby Jesus came from the body of Mary. Taking His human body from her would have been a desecration of the Son of God if she had been tainted in any way with the sin of Adam
It is important to note that the Immaculate Conception is not a newly minted teaching of the Church. The Modernists would have you believe that Church teaching can be changed, created, or done away with at the whim of the hierarchy—but this is utterly false, for matters of Faith and morals are matters of Truth and therefore incapable of change. While the Immaculate Conception was declared a doctrine of the Faith as recently as 1854, it is not a new teaching—it has been held by the great theologians and doctors of the Church since the earliest days.
If there has been any dispute about Mary's preservation from sin it has been a question of timing. Medieval science did not understand that life begins at Conception. The medieval theory was that the human embryo was like a seed planted in the ground that required some time to sprout. If this were true, Mary's sanctification might have been delayed by days or even weeks! Theology, combined with modern science, tells us that Mary’s preservation from original sin was instantaneous.
The timeless nature of God allowed Mary to benefit in advance from the redemption their Son would work in time.
Even though there was fairly general agreement about this dogma of the Immaculate Conception, any discussion was resolved conclusively in the 19th century.
Our Lady appeared to Catherine Labouré, a Sister of Charity in Paris, on November 27th, 1830, and asked that a medal be struck in honor of the Immaculate Conception. Most all of us have seen it—we blessed them the other day on the 27th, the feast day—what we know as the Miraculous Medal, bearing the inscription: “Mary, conceived without sin, pray for us who have recourse to thee.”
Twenty four years later, on December 8th, 1854 our holy father, Blessed Pope Pius IX, solemnly defined the Immaculate Conception as a doctrine of the Faith ... “that the most blessed Virgin Mary, at the first instant of her conception . . . was preserved from any stain of original sin” ... something to which we must assent if we wish to call ourselves Catholics.
And a few years later, our Lady herself again confirmed the message of this dogma, when she told St. Bernadette of Lourdes, “I am the Immaculate Conception.”
If we work backwards, we will see that the sixteenth century Council of Trent specifically excluded the Blessed Mother from its discussion of Original Sin, for it was clear that it had no effect on her. In the fifteenth century, Pope Sixtus IV extended the liturgical feast to the City of Rome and condemned all those who judged the Immaculate Conception to be an erroneous teaching.
In the fourteenth century John I, King of Aragon named the Immaculate Virgin his own and his country’s protector. The feast of Mary’s Conception was celebrated in Belgium in the twelfth century; in Germany and France in the eleventh, and in England at the time of the Norman Conquest; it was celebrated at the court of Charlemagne in the ninth century with a hymn composed by Paul the Deacon; celebrated at Naples in the ninth century and in Spain in the eighth. In sixth century Palestine the feast appears in liturgical book of Saint Sabas, whose feast fell but a few days ago. Late in the fourth or very early in the fifth century, the “spotless virgin ... triumphing over the poisons of Satan” was the subject of the poet Aurelius Clemens Prudentius’ (348-405) hymn Ante cibum.
In roughly the same time period, Saint Germain wrote of Mary, the “most pleasing spiritual paradise of God ... that fragrant lily, that unfading rose who heals those who drank the soul-killing bitterness of death ... paradise wherein grows the tree of the knowledge of truth, the tree that gives immortality to those who taste its fruit ... most pure palace of God the supreme King.”
Saint Jerome, writing at about the same time as Prudentius was characteristically blunt. “The virtue and greatness of the blessed Mary ever Virgin were proclaimed in God’s own words by the Angel when he said, «Hail, full of grace, the Lord is with thee. Blessed art thou among women»... whereas grace is given to others only in part, on Mary it was poured out all at once in all its fullness.... everything about her is wholly the work of purity and simplicity, of grace and truth, of the mercy and justice that look down from heaven.”
Simply put, when the Angel Gabriel appeared to Mary to announce her divine Motherhood, it was at a time in history when it was impossible for anyone to be filled with grace unless she had always been that way—from the time of her conception!
Finally, it is fitting, then, to pray with the Church today to almighty God:
"O God who, by the Immaculate Conception of the Virgin didst make her a worthy habitation for Thy Son, and didst, by His foreseen death, preserve her from all stain of sin; grant, we beseech Thee, that through her intercession we may be cleansed from sin and come with pure hearts to Thee. Through the same our Lord....”
 Genesis iii:15 http://www.drbo.org/x/d?b=drl&bk=1&ch=3&l=15-#x
 Ineffabilis Deus, Apostolic Constitution of Pope Pius IX, December 8, 1854.
 Council of Trent, Session V, 17 June 1546, “Decree on Original Sin” para. 6. (Denzinger 792)
 Sixtus IV, “Cum præexcelsa,” 28 February 1476 and “Grave nimis” 4 September 1483. (Denzinger 434 & 435)
 Dom Guéranger, The Liturgical Year, book I “Advent,” pp. 383-386, 408-490.
 St. Germain, “On the Presentation of the Mother of God.”
 Lessons iv and v at Matins of the Immaculate Conception, Saint Jerome, “On the Assumption of the Blessed Virgin Mary.”