Exodus 34:29 tells us that when God wrote the words of the Law on the stone tables and gave them to Moses, "the skin of his face shone because of his speaking with" God. Moses' face shone so greatly from His Presence that he was obliged to wear a veil, to avoid blinding his terrified countrymen who gathered at the base of the mountain. 2 Samuel 6:7 reminds us that when these words were placed in a vessel of acacia wood, the resultant Ark of the Covenant was so unimaginably Holy that to touch it meant instant death.
With such passages in mind, what should we then think of the Jewish girl in whose womb the immortal Word (from whom the words of the Law had been derived) was made present in the flesh? What words can possibly begin to express the holiness of this, the Mother of God, the Ark of the New Covenant?
Catholics understand two categories of honoring a person or thing. Latria is the worship and adoration due solely to God, and dulia the respect and veneration proper to created things. Mary obviously falls under the latter category, and yet she outranks almost everything else. No prior consecration of the patriarchs, the judges, the kings, the prophets, or the nation of Israel itself can compare to the honor and distinction bestowed on Mary, the Mother of God. She is utterly unique in Scripture. It is for this reason that Catholics consider Marian devotion to be hyperdulia -- proper to a created being, but beyond what would be accorded to almost anything else.
Even the angels accorded her exceptional honors. When the archangel Gabriel appeared at the Annunciation, the closest literal translation of his greeting is: "Hail, full of grace, the Lord with you." This is why "she was very perplexed at this statement, and kept pondering what kind of salutation this was" (Luke 1:28-29). Mary may be the only individual who isn't immediately struck dumb at the sight of an angel, and for whom the immediate reassurance of "Do not be afraid" isn't actually necessary. She is certainly the only one to be greeted by a title rather than a name.
The title itself is a linguistic mouthful. The original text, in Greek, is kecharitomene, which is the perfect passive participle of charitoo (grace, blessing, favor). The perfect tense has a pretty specific meaning in Greek: it connotes a present state of completeness, contingent upon a past action. Thus the phrase "it is written" uses the perfect tense: it was written in the past, but brought to perfection in the present (in the person of Jesus Christ). For those who want to research further, I found this article to be particularly valuable. Essentially, the salutation kecharitomene indicates a state of unique and perfected grace which had already been bestowed upon Mary and which would now be brought into completeness.
Confused? So was Mary.
To understand this unique grace which had been bestowed, and was about to be brought to fruition, it's necessary to understand another typology with which Mary is usually associated. Jesus Christ often referred to Himself as "the Son of Man" and is often compared to the first Man, Adam. Indeed, Romans 5:14 explicitly states that "Adam... is a type of Him who was to come" -- that is, that Adam is a typology of Christ. "For as in Adam all die, so also in Christ all will be made alive" (1 Cor. 15:22).
Just as Adam was a type for Jesus Christ, so the early Church treated Eve as a type for Mary
Consider again the significance of the Annunciation. God sought to come into flesh, to incarnate Himself and redeem humanity as Man. But this could not occur without the consent of Mary. By the very principle of free will that had caused Adam's fall in the first place, the divine plan of redemption was placed at the mercy of the young Jewish girl from Nazareth.
When Adam sinned, it was Eve who preceded him. Her sin led to his Fall. Thus, by that same typology, it would be the second Eve's faithfulness that would anticipate the second Adam's sacrifice. Her obedience at the Annunciation would enable His obedience in Gethsemane, even unto death.
Can we begin to fathom the cosmic pyrotechnics that accompanied her response to Gabriel? "Behold, the bond-servant of the Lord; may it be done to me according to your word." Her justly-celebrated fiat is one of the defining moments of Creation: the pinnacle of all the patriarchs, of all the prophets, and all the people of God. By the free will of the first Adam, man separated himself from the Presence of God. Now, by the free will of the second Eve, God was made Present to again walk among men. This is the meaning of St. Jerome's dictum: "Death came through Eve, but Life has come through Mary."
This typology should explain Gabriel's salutation of Mary. She was full of grace, given in anticipation of this very moment. Eve had chosen the apple of her own free will, in a state of innocence. In this moment, this second chance for humanity, Mary was endowed with the same grace and freedom.
This is the Catholic dogma of the Immaculate Conception. Protestants, take note: the Immaculate Conception refers to the conception of Mary, not the conception of Jesus (an event celebrated as the Annunciation or the Incarnation). I'm rather ashamed at how long it took me to recognize the distinction.
The history of this dogma is particularly interesting. Though it was defined in 1854, the teaching that Mary had by the grace of God lived a sinless life had been held long before that time, arising from the early Church and recognized by both Roman Catholic and Eastern Orthodox Christians. The reason it took so long to define dogmatically is because, for most of its history, this teaching appeared exclusively in the liturgy of the Church, and wasn't discussed in academic settings.
When Eadmer, companion to St. Anselm, published a tract on the subject in the early eleventh century, the long-standing teaching moved from the realm of doxology to the field of theology. This was especially the case at the University of Paris, where the doctrine was subject to heated debates between the Dominicans (best represented by the Angelic Doctor, St. Thomas Aquinas) and the Franciscans (notably the Blessed John Duns Scotus).
The debate focused primarily on three questions. First, in what way was Mary sinless? Was she preserved from "original sin" (sin nature in toto) or merely from "actual sin" (that is, sinful deeds)? It is for this reason that the Eastern Orthodox Church recognizes Mary as "spotless" without agreeing to the Immaculate Conception as Catholics define it. Second, what were the consequences of this sinlessness? This discussion mostly focused on whether Mary was subject to the usual sufferings of humanity -- especially the labor pains of childbirth -- and whether she ever entertained sinful thoughts or temptations.
The third question, however, was the bigger point of contention: at what point did her sinlessness take effect? Virtually everyone agreed that this sanctifying grace was endowed to Mary before her birth, but there was a good deal of controversy whether it was given at the moment of physical conception, or some indeterminate time after conception but before birth. Ultimately, the scholarly consensus arrived at a middle position. Relying on the insights of medieval scholasticism, and in contrast to the sometimes dualistic approach of the post-Enlightenment West, Catholic theology defines man as a holistic union of body and soul. For this reason, Catholic theology distinguishes between "conception" simpliciter, in the purely physical sense, and "conception" in the broader sense, the moment at which the body is fused with a rational soul. Mary's immaculate conception is therefore interpreted as applying to the latter (sometimes called "animation"), since original sin is transmitted by flesh but pertains to our souls.
Once this consensus was reached, the dogma was solemnly defined by the Council of Basel, held between 1431–1449, and was recognized by all Catholic theologians. However, some time later it was determined that the Council of Basel was not a proper ecumenical council according to canon law, and therefore unable to promulgate dogma, and so the question was left up in the air for some time. It wasn't until the mid-nineteenth century that, with the nearly unanimous consent of the College of Cardinals and the bishops of the Catholic Church, Pope Pius IX defined the dogma in his apostolic constitution of 1854, Ineffabilis Deus:
We declare, pronounce and define that the doctrine which holds that the Blessed Virgin Mary, at the first instant of her conception, by a singular privilege and grace of the Omnipotent God, in virtue of the merits of Jesus Christ, the Saviour of mankind, was preserved immaculate from all stain of original sin, has been revealed by God, and therefore should firmly and constantly be believed by all the faithful.By the atoning blood of Jesus Christ shed on the cross, Mary was conceived and born free of original sin. This was not accomplished out of her own power, for she was still in need of a Savior (Luke 1:47), but was accomplished by the timeless and eternal God, in anticipation of the momentous choice she would face.
The dogma of the Immaculate Conception is considered theologically necessary not only in light of her role as the New Eve, but also in light of her role as Theokotos, Mother of God. Christians profess that Jesus Christ was a single person with two natures, human and divine. His divine nature was inherited as the only begotten Son of God. His human nature was inherited from His conception in the womb of Mary. If Mary existed in a state of original sin, that stain would have been inherited through the flesh by her son Jesus.
We might hypothesize that Jesus was Himself the subject of the sanctifying grace, to be preserved from original sin, but that would be tautological: could Jesus' blood have preserved His own state of sinlessness in vitro? Moreover, such a teaching would greatly diminish His humanity, since He would only be human in a cleansed and rarefied manner.
Rather, the Church teaches that God preserved Mary from conception in a state of innocence -- though it should be noted that this preservation was not yet perfection. By allowing her the chance to recapitulate Eve's fateful decision, to triumph in the face of temptation, Mary's humanity was glorified, as human nature would have been but for the Fall. It was this amplified and renewed form of human nature -- fully endowed with the sanctifying grace of God and perfected in the cooperative merit of Mary's faithful fiat -- that was inherited by Jesus Christ.