WWJD
What the Catholic Church means by the Immaculate Conception is that Mary, while conceived the ordinary way, was preserved from the stain of original sin by a special grace of God in anticipation of the death of Jesus Christ. Or simply, Mary had to be free of original so she could be the Mother of God and that the grace that preserved her from original sin was from the death of Jesus Christ (but in heaven, where there is no time but one Present. Jesus is always "The Lamb that seemed to have been slain" described in Revelation 5:6). Mary is a great example of what the Bible means by saying that each one of us is a "vessel" (Romans 9:20-21 and 2 Timothy 2:20-21). These verses also show that God exercises his Lordship over us when he "molds" us, showing that God can indeed intervene on someone's behalf with a special act of grace like in the Immaculate Conception. Is not Mary then the vessel that nurtured and gave birth to Jesus? The question is whether or not Mary had to be pure.
The basis for the belief in the Immaculate Conception of Mary can be found in the Biblical revelation of holiness and the opposite of that state, sinfulness.
God is revealed as perfect interior holiness.
Is 6:3 "Holy, holy, holy is the Lord of hosts!" they (the Seraphim) cried one to the other.
No sin or anything tainted with sin can stand in the face of the holiness of God. "Enmity" is that mutual hatred between Mary and sin, between Christ and sin.
Gen 3:15 I will put enmity between you (the serpent, Satan) and the woman (Mary), and between your offspring (minions of Satan) and hers (Jesus); He will strike at your head, while you strike at his heel.
For the birth of God as a human being, God was interested in the condition of the mother's womb. For even a great, but imperfect, judge of Israel, Samson, God was directive about the state of the mother during the pregnancy. The request for the mother to be pure is repeated for emphasis.
Judges 13:3-4 An angel of the LORD appeared to the woman and said to her, "Though you are barren and have had no children, yet you will conceive and bear a son. Now, then, be careful to take no wine or strong drink and to eat nothing unclean."
Judges 13:7 "But he (the angel) said to me, 'You will be with child and will bear a son. So take neither wine nor strong drink, and eat nothing unclean. For the boy shall be consecrated to God from the womb, until the day of his death.' "
Judges 13:13-14 The angel of the LORD answered Manoah, "Your wife is to abstain from all the things of which I spoke to her. She must not eat anything that comes from the vine, nor take wine or strong drink, nor eat anything unclean. Let her observe all that I have commanded her."
How much more would God be interested in the state of His own mother's womb!
The salutation of the Angel Gabriel is different from the usual angelic greeting. It indicates that Mary was exceptionally "highly favored with grace" (Greek: charitoo, used twice in the New Testament, in Lk 1:28 for Mary - before Christ's redemption; and Eph 1:6 for Christ's grace to us - after Christ's redemption).
Lk 1:28 And coming to her (Mary), he (the angel Gabriel) said, "Hail, The One Full of Grace (kecharitomene)"
Eph 1:4-6 (God) chose us in him (Jesus), before the foundation of the world, to be holy and without blemish before him. In love he destined us for adoption to himself through Jesus Christ, in accord with the favor of his will, for the praise of the glory of his grace (echaritosen) that he granted us in the beloved.
Note that the angel's salutation preceded Mary's acquiescence. Mary was already Full of God's Grace. God's grace was not given in time after Mary accepted the angel's word. The Church believes that this grace was given from the very beginning of Mary's life. It is clearly grace because at the time of Mary's conception she could have done nothing to earn it.
The constant faith (paradosis) of the Church attests to the belief in the special preparation of the holiness of the person of Mary to bear in her body the most holy person of the Son of God.
Deuteronomy 23 is a chapter that shows that under the Old Covenant, (which would be in effect until the moment of Christ's death) that in order for a person to enter the synagogue, that person must have the stain of generational sin removed. Even if that person lives a perfectly holy life, that person couldn't enter the synagogue. Would not the same rules apply to Our Lord who, after he ascended into heaven immediately sat down at the right hand of the Father? Let me remind you that nothing unclean shall ever enter into heaven (Revelation 21:27). How could he do that if He was the first offspring of a mother who was stained with original sin? We know that Jesus' baptism isn't the answer since his baptism was to let all of Israel know that he indeed was the Messiah and to be equipped for His ministry by the Holy Spirit (Christ, of course would be in no need of being baptized for the remission of sins) . John the Baptist in fact knew that he was free from original sin for he said, "I need to be baptized by you, and yet you are coming to me?" (Matthew 3:14).
Would it not be reasonable, then to conclude that by necessity Mary had to be free from the stain of all sin so that Jesus could be? In other words, would it not make sense to keep a container of food clean so it wouldn't contaminate the food stored in it? Jesus' humanity came from Mary, and we know that Jesus' humanity is perfect. Secondly, 2 Corinthians 6:14 asks us, "For what partnership do righteousness and lawlessness have? Or what fellowship does light have with darkness?" If indeed Mary was conceived in the state of original sin, then it would be difficult to imagine what kind of "fellowship" her relationship with her Son would be.
But what about Mary's own statement: “My soul glorifies the Lord and my spirit rejoices in God my Savior..."(Luke 1:46-47)?
Some will say; 'see, even Mary says she needed a Savior, she cannot have been without sin'.
AMEN, Mary is human and needed a Savior. The answer lies in HOW she is saved. Think of sin like a mud puddle. When we are born, we fall into the puddle. We are saved when God, by His Grace, pulls us out and cleans us up. Mary was saved by God, by His Grace, keeping her from falling into the mud in the first place.
It is sad that so many that quote vs 46-47 of Luke, then forget vs 48-49:
"From now on all generations will call me blessed, for the Mighty One has done great things for me—Holy is His name.