It’s important to understand what the doctrine of the Immaculate Conception is and what it is not. Some people think the term refers to Christ’s conception in Mary’s womb without the intervention of a human father; but that is the Virgin Birth. Others think the Immaculate Conception means Mary was conceived "by the power of the Holy Spirit," in the way Jesus was, but that, too, is incorrect. The Immaculate Conception means that Mary, whose conception was brought about the normal way, was conceived without original sin or its stain—that’s what "immaculate" means: without stain. The essence of original sin consists in the deprivation of sanctifying grace, and its stain is a corrupt nature. Mary was preserved from these defects by God’s grace; from the first instant of her existence she was in the state of sanctifying grace and was free from the corrupt nature original sin brings.
"When discussing the Immaculate Conception, an immediant reference may be found in the angel’s greeting to Mary. The angel Gabriel said, "Hail, full of grace, the Lord is with you" (Luke 1:28). The phrase "full of grace" is a translation of the Greek word kecharitomene. It therefore expresses a characteristic quality of Mary.
The traditional translation, "full of grace," is better than the one found in many recent versions of the New Testament, which give something along the lines of "highly favored daughter." Mary was indeed a highly favored daughter of God, but the Greek implies more than that (and it never mentions the word for "daughter"). The grace given to Mary is at once permanent and of a unique kind. Kecharitomene is a perfect passive participle of charitoo, meaning "to fill or endow with grace." Since this term is in the perfect tense, it indicates that Mary was graced in the past but with continuing effects in the present. So, the grace Mary enjoyed was not a result of the angel’s visit. In fact, Catholics hold, it extended over the whole of her life, from conception onward. She was in a state of sanctifying grace from the first moment of her existence. "
Fundamentalists’ chief reason for objecting to the Immaculate Conception and Mary’s consequent sinlessness is that we are told that "all have sinned" (Rom. 3:23). Besides, they say, Mary said her "spirit rejoices in God my Savior" (Luke 1:47), and only a sinner needs a Savior.
Let’s take the second idea first. Mary, too, required a Savior. Like all other descendants of Adam, she was subject to the necessity of contracting original sin. But by a special intervention of God, undertaken at the instant she was conceived, she was preserved from the stain of original sin and its consequences. She was therefore redeemed by the grace of Christ, but in a special way—by anticipation.
An analogy I stumbled upon sums it up as: Suppose a man falls into a deep pit, and someone reaches down to pull him out. The man has been "saved" from the pit. Now imagine a woman walking along, and she too is about to topple into the pit, but at the very moment that she is to fall in, someone holds her back and prevents her. She too has been saved from the pit, but in an even better way: She was not simply taken out of the pit, she was prevented from getting stained by the mud in the first place. This is the illustration Christians have used for a thousand years to explain how Mary was saved by Christ. By receiving Christ’s grace at her conception, she had his grace applied to her before she was able to become mired in original sin and its stain.
The Catechism of the Catholic Church states that she was "redeemed in a more exalted fashion, by reason of the merits of her Son" (CCC 492). She has more reason to call God her Savior than we do, because he saved her in an even more glorious manner! But what about Romans 3:23, "all have sinned"? Have all people committed actual sins? Consider a child below the age of reason. By definition he can’t sin, since sinning requires the ability to reason and the ability to intend to sin.
We also know of another very prominent exception to the rule: Jesus (Heb. 4:15). So if Paul’s statement in Romans 3 includes an exception for the New Adam (Jesus), one may argue that an exception for the New Eve (Mary) can also be made.
Paul’s comment seems to have one of two meanings. It might be that it refers not to absolutely everyone, but just to the mass of mankind (which means young children and other special cases, like Jesus and Mary, would be excluded without having to be singled out). If not that, then it would mean that everyone, without exception, is subject to original sin, which is true for a young child, for the unborn, even for Mary—but she, though due to be subject to it, was preserved by God from it and its stain.
The objection is also raised that if Mary were without sin, she would be equal to God. In the beginning, God created Adam, Eve, and the angels without sin, but none were equal to God. Most of the angels never sinned, and all souls in heaven are without sin. This does not detract from the glory of God, but manifests it by the work he has done in sanctifying his creation. Sinning does not make one human. On the contrary, it is when man is without sin that he is most fully what God intends him to be.
The doctrine of the Immaculate Conception was officially defined by Pope Pius IX in 1854. When Fundamentalists claim that the doctrine was "invented" at this time, they misunderstand both the history of dogmas and what prompts the Church to issue, from time to time, definitive pronouncements regarding faith or morals. They are under the impression that no doctrine is believed until the pope or an ecumenical council issues a formal statement about it.
Actually, doctrines are defined formally only when there is a controversy that needs to be cleared up or when the magisterium (the Church in its office as teacher; cf. Matt. 28:18–20; 1 Tim. 3:15, 4:11) thinks the faithful can be helped by particular emphasis being drawn to some already-existing belief. The definition of the Immaculate Conception was prompted by the latter motive; it did not come about because there were widespread doubts about the doctrine. In fact, the Vatican was deluged with requests from people desiring the doctrine to be officially proclaimed. Pope Pius IX, who was highly devoted to the Blessed Virgin, hoped the definition would inspire others in their devotion to her.
-I've recieved some questions regarding this topic, and I put this big post together about it. I hope this answers their questions and others. Now, don't think this is all my writting, some is, and some is others. I do not take credit for this. =D