THE DOGMA
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin. -Pope Pius IX, Ineffabilis Deus, 1854 -Ex Cathedra
Read Catechism of the Catholic Church paragraphs 488-494
 
THE BIBLICAL BASIS OF THE IMMACULATE CONCEPTION
The Immaculate Conception was foretold in Genesis 3:15;
I will put enmity between you (satan/sin) and the woman (Mary), and between your offspring (sin) and hers (Jesus); He will strike at your head, while you strike at his heel." 
We know that Jesus was conceived without sin (original sin and personal sin). In the verse above, it mentions the enmity of sin and Jesus & the enmity of sin and Mary. Because both are mentioned in the same sentence and described in the same way, the enmity is the same. If the enmity between Mary and sin were not the same as the enmity between Jesus and sin they would not have been included in the same sentence. If there is a complete enmity between Christ and sin there must be a complete enmity between Mary and sin. Thus, Jesus and Mary were born without sin. The enmity between Mary and sin (at conception) is the Immaculate Conception.
Another reference to the Immaculate Conception in the Bible is done by the Angel Gabriel in Luke 1:28;
And coming to her, he said, "Hail, favored one! The Lord is with you." (NAB Bible)
And the angel being come in, said unto her: Hail, full of grace, the Lord is with thee: blessed art thou among women. (Douay Rheims Bible)
The greeting of Gabriel to Mary was "Χαῖρε, κεχαριτωμένη/Chaire, kecharitomene (Hail, full of grace!)" To be "full of grace" you would have to be empty of sin. Back then it was impossible to be sinless except through divine privilege. The only way to be "full of grace" is after receiving the sacraments of baptism and confession. But the annunciation occurred before baptism and confession were even instituted. At that time the only way to be "full of grace" is through a divine privilege. 

Note that the angel's salutation came before being acquainted with Mary. Mary was already full of grace. God's grace was not given in time after Mary accepted the angel's word. God's grace was given from the very beginning of Mary's life.


In addition, Mary is the New Testament Ark of the Covenant. The Old Testament Ark of the Covenant was pure and spotless inside and out. 
 
THE CHURCH FATHERS ON THE IMMACULATE CONCEPTION
Hippolytus - "He was the ark formed of incorruptible wood. For by this is signified that His tabernacle was exempt from putridity and corruption." -Orations Inillud, Dominus pascit me (ante A.D. 235).
Origen - "This Virgin Mother of the Only-begotten of God is called Mary, worthy of God, immaculate of the immaculate, one of the one." -Homily 1(A.D. 244).
Ephraem - "Let woman praise Her, the pure Mary." -Hymns on the Nativity, 15:23 (A.D. 370).
Ephraim the Syrian "Thou alone and thy Mother are in all things fair, there is no flaw in thee and no stain in thy Mother." -Nisibene Hymns, 27:8 (A.D. 370).
Athanasius - "O noble Virgin, truly you are greater than any other greatness. For who is your equal in greatness, O dwelling place of God the Word? To whom among all creatures shall I compare you, O Virgin? You are greater than them all O Covenant, clothed with purity instead of gold! You are the Ark in which is found the golden vessel containing the true manna, that is, the flesh in which divinity resides." -Homily of the Papyrus of Turin, 71:216 (ante AD 373).
Ambrose of Milan - "Mary, a Virgin not only undefiled but a Virgin whom grace has made inviolate, free of every stain of sin." -Sermon 22:30 (A.D. 388).
Augustine - "We must except the Holy Virgin Mary, concerning whom I wish to raise no question when it touches the subject of sins, out of honour to the Lord; for from Him we know what abundance of grace for overcoming sin in every particular was conferred upon her who had the merit to conceive and bear Him who undoubtedly had no sin." -Nature and Grace,4 2[36] (A.D.415).
Proclus of Constantinople - "As he formed her without my stain of her own, so He proceeded from her contracting no stain." -Homily 1 (ante A.D. 446).
Theodotus of Ancrya - "A virgin, innocent, spotless, free of all defect, untouched, unsullied, holy in soul and body, like a lily sprouting among thorns." -Homily 6:11(ante A.D. 446).
Peter Chrysologus - "The angel took not the Virgin from Joseph, but gave her to Christ, to whom she was pledged from Joseph, but gave her to Christ, to whom she was pledged in the womb, when she was made." -Sermon 140 (A.D. 449).
Jacob of Sarug - "[T]he very fact that God has elected her proves that none was ever holier than Mary, if any stain had disfigured her soul, if any other virgin had been purer and holier, God would have selected her and rejected Mary." (A.D. 521).
Theotokos of Livias - "She is born like the cherubim, she who is of a pure, immaculate clay." -Panegyric for the feast of the Assumption, 5:6 (ante A.D. 650).
Andrew of Crete - "Today humanity, in all the radiance of her immaculate nobility, receives its ancient beauty. The shame of sin had darkened the splendour and attraction of human nature; but when the Mother of the Fair One par excellence is born, this nature regains in her person its ancient privileges and is fashioned according to a perfect model truly worthy of God.... The reform of our nature begins today and the aged world, subjected to a wholly divine transformation, receives the first fruits of the second creation." -Sermon I, On the Birth of Mary (A.D. 733).
Germanus of Constantinople - "[T]ruly elect, and superior to all, not by the altitude of lofty structures, but as excelling all in the greatness and purity of sublime and divine virtues, and having no affinity with sin whatever." -Marracci in S. Germani Mariali (ante A.D. 733).
John of Damascus - "O most blessed loins of Joachim from which came forth a spotless seed! O glorious womb of Anne in which a most holy offspring grew." -Homily I (ante A.D. 749).
Justin Martyr - [Jesus] became man by the Virgin so that the course that was taken by disobedience in the beginning through the agency of the serpent might be also the very course by which it would be put down. Eve, a virgin and undefiled, conceived the word of the serpent and bore disobedience and death. But the Virgin Mary received faith and joy when the angel Gabriel announced to her the glad tidings that the Spirit of the Lord would come upon her and the power of the Most High would overshadow her, for which reason the Holy One being born of her is the Son of God. And she replied, "Be it done unto me according to your word." (Luke 1:38) -Dialog with Trypho 100 (A.D. 155).
Irenaeus - Consequently, then, Mary the Virgin is found to be obedient, saying, "Behold, 0 Lord, your handmaid; be it done to me according to your word." Eve . . . who was then still a virgin although she had Adam for a husband — for in paradise they were both naked but were not ashamed; for, having been created only a short time, they had no understanding of the procreation of children . . . having become disobedient [sin], was made the cause of death for herself and for the whole human race; so also Mary, betrothed to a man but nevertheless still a virgin, being obedient [no sin], was made the cause of salvation for herself and for the whole human race. . . . Thus, the knot of Eve's disobedience was loosed by the obedience of Mary. What the virgin Eve had bound in unbelief, the Virgin Mary loosed through faith. -Against Heresies 3:22:24 (A.D. 189).
Origen - This Virgin Mother of the Only-begotten of God is called Mary, worthy of God, immaculate of the immaculate, one of the one -Homily 1 (A.D. 244).
Hippolytus - He [Jesus] was the ark formed of incorruptible wood. For by this is signified that His tabernacle [Mary] was exempt from defilement and corruption -Orat. In Illud, Dominus pascit me, in Gallandi, Bibl. Patrum, II, 496 ante (A.D. 235).
Ephraim the Syrian - You alone and your Mother are more beautiful than any others, for there is neither blemish in you nor any stains upon your Mother. Who of my children can compare in beauty to these? -Nisibene Hymns 27:8 (A. D. 361).
Ambrose of Milan - Come, then, and search out your sheep, not through your servants or hired men, but do it yourself. Lift me up bodily and in the flesh, which is fallen in Adam. Lift me up not from Sarah but from Mary, a Virgin not only undefiled but a Virgin whom grace had made inviolate, free of every stain of sin. -Commentary on Psalm 118:22-30 (A.D. 387).
Gregory Nazianzen - He was conceived by the virgin, who had been first purified by the Spirit in soul and body; for, as it was fitting that childbearing should receive its share of honor, so it was necessary that virginity should receive even greater honor. (Sermon 38 d. A.D. 390).
Theodotus of Ancrya - A virgin, innocent, spotless, free of all defect, untouched, unsullied, holy in soul and body, like a lily sprouting among thorns -Homily 6:11(ante A.D. 446).
Romanos the Melodist - Then the tribes of Israel heard that Anna had conceived the immaculate one. So everyone took part in the rejoicing. Joachim gave a banquet, and great was the merriment in the garden. He invited the priests and Levites to prayer; then he called Mary into the center of the crowd, that she might be magnified. -On the Birth of Mary 1 (d. ca A.D. 560).


    RESPONSE TO ARGUMENT OPPOSING THE IMMACULATE CONCEPTION
    "Romans 3:23 clearly says 'All have sinned. . . .' The dogma of the Immaculate Conception is clearly contradicting the Bible!"
    The meaning of the verse lies in context. St Paul is telling the Jewish Christians that they have no supremacy over Gentile Christians. He is saying "You are both equal! All have sinned." If the meaning of the verse was strict, it would have to include babies, the mentally disable, and even Jesus! All have sinned except for Jesus and Mary. This is said in Genesis 3:15.
    It is possible that Paul was talking about mankind in general. And that Jesus, Mary, infants, and the mentally disabled would be excluded but not singled out. He might have also meant that everyone is subject to original sin (except Jesus). Unborn babies, infants, mentally disabled people, and even Mary are subject to original sin. Mary was subject to original sin, but through God's grace she was preserved from it.
    "Mary is not equal to God! No one is equal to God!"
    Catholics agree with the statement above 100%! No one is equal to God!! Adam, Eve, and the angels were created without sin (personal and original sin) yet they are not equal to God! The Angels and Saints in Heaven are without sin and are not equal to God!
    "What about St Stephen? The Bible says that he was "filled with grace" (Acts 6:8), but why wasn't he immaculately conceived?"
    St Stephen was already baptized, and therefore he could easily have been as "full of grace" as Mary at that point (through confession or baptism). But he was not "full of grace" from conception.
    "The Bible says that Mary needed a savior (Luke 1:47). Only sinners need a savior!"
    Let’s take the second citation 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.
    Consider an analogy: 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. (Catholic Answers)
    "The Greek word "κεχαριτωμένη/kecharitomene" could also mean "full of honor", this is referring to the honor of being the mother of Jesus!"
    "Kecharitomene" is a perfect passive participle of charitoo, meaning "to fill or endow with grace/special honor." 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/honor Mary had is not because she was the mother of Jesus (Jesus was not yet conceived (Luke 1:31,35)) or being visited by an angel. If the greeting was referring to a special honor, the greeting would have been reffering to the Immaculate Conception. The only special honor that Mary received was the Immaculate Conception.

    Martin Luther on the Immaculate Conception
    "But as the Virgin Mary was herself born of a father and mother in the natural way, many have been disposed to assert that she was also born in original sin, though all with one mouth affirm that she was sanctified in the maternal womb, and conceived without concupiscence." (On the Day of the Conception of the Mother of God).