From 2018: Apologia for the Immaculate Conception

 Here was something I wrote (warning: it's long) for a friend who had someone challenge her on Mary's Immaculate Conception and sinlessness. Her friend's challenge came from citing Romans 3:23, so the first part addresses that, then the rest goes into the Biblical confirmation of Mary's sinlessness:

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So, there's no short way to answer this. First we have to look at the Romans 3:23 passage "all have sinned and are deprived of the glory of God." (NAB), and then we look at Scriptural references to Mary's sinlessness. 

First regarding the passage your friend quoted you could, in turn, point out Hebrews 4:15 which says, in reference to Jesus, "For we do not have a high priest who is unable to sympathize with our weaknesses, but one who has similarly been tested in every way, yet without sin." So, looking back at Romans, not EVERYONE has sinned, otherwise you'd have to include Jesus or deny his humanity. So we can't use Romans 3:23 to definitively say all individuals have sinned.

Paul in Romans 3:23 is using a type of hyperbole in that he's not talking about ALL as individuals, but ALL as in all humanity under Adam's headship, emphasizing, and this is Paul's key point, Gentiles AND Jews have sinned despite the Jews having the Law. You can see that he's talking about ALL in reference to the two groups in the same chapter 3:9 "we have already brought the charge against Jews and Greeks alike that they are all under the domination of sin" Paul's driving argument throughout Romans is that the Law is no more a means to salvation than what philosophical notions of God or morality the Gentiles had figured out on their own. Paul sums this up in the previous chapter 2:12 "All who sin outside the law [Gentiles] will also perish without reference to it, and all who sin under the law [Jews] will be judged in accordance with it."

So, that's the first part, to point out that Paul in Romans is not talking individuals when he says "all," but Jews and Gentiles collectively. So you can't legitimately use that verse to definitively say Mary DID sin just because she's human. ('cause again, if it meant all individually, that would have to include Jesus Christ or deny his humanity)

Second, addressing specifically that Mary was sinless, we have to do a little more work; there is no specific verse one could point to in Scripture that would definitively state that Mary was without sin. But you can't point out any scripture verse that says that specifically state that God is one nature and three persons, or that the three persons Father, Son and Holy Spirit are co-equal and co-eternal, either. There is also no scriptural verse that definitively says that Jesus was fully God and fully human. Those ancient teachings of the Church, and unchangeable beliefs of those who call themselves Christian, are derived from the way the Church worshiped the Three Persons from the beginning (eg "baptize them in the name of the Father, Son, and Holy Spirit") and confirmed by carefully study of the full presentation of God and Jesus in the New Testament, despite no individual verses stating so.

First, it's important to know and tell people the Church does not believe that Mary was free from sin BY HER OWN MERIT. Mary was saved from sin by Jesus Christ through His dying on the Cross at the moment of her conception. Catholics believe that because the Crucifixion is an act of God, the saving merit of our redemption is not bound by time, just as God is not bound by time. This is why before the Crucifixion we see Moses and Elijah with Jesus in his Transfiguration, and the beggar Lazarus can speak of being in Abraham's bosom as analogous to heaven (implying that Abraham is already in heaven, pre-Crucifixion). No one comes to heaven except by God's saving action on the Cross, and yet these Old Testament figures are shown as with God before the Crucifixion, so we know that the Cross is not bound by time in its saving grace. 

Likewise, scripture states that God can save us FROM sinning as per the Letter of Jude: Now to him who is able to keep you from falling and to present you without blemish before the presence of his glory with rejoicing, to the only God, our Savior through Jesus Christ our Lord, be glory, majesty, dominion, and authority, before all time and now and for ever. (Jude 24-25) So Mary could have been saved by God alone, from sin at the time of her conception ("who is able to keep you from falling"), though her her conception and most of her life happened before the Crucifixion. But, stating she could have been saved from sin isn't the same as saying she was, in fact, saved from sin, or that it would have been proper of Jesus to do so.

Now like the doctrine of the Trinity and Jesus two natures being confirmed by scripture, there are scriptural verses that confirm Mary's sinlessness, though some strongly support it by analogy and not with direct reference to her. There is one verse, directly addressed about Mary that when deeply studied for Catholics confirms her sinlessness: the angel Gabriel's address to Mary, "Hail, full of grace." (NAB) "Hail, thou that art highly favoured" (KJV) ""Greetings, you who are highly favored!" (NIV) 

The Greek word that is above is translated as "full of grace" etc is (and now I'm quoting from another source): 

"kekaritomene, [...] This word literally means "she who has been graced" in a completed sense. This verbal adjective, "graced," is not just describing a simple past action. Greek has another tense for that. The perfect tense is used to indicate that an action has been completed in the past resulting in a present state of being."

It is a word unique in Scripture, applied to no one else. In fact, Paul, for example, when he writes about Christians, always speaks of them as being works in progress. But not so with Mary, she is unique in that the angel's greeting means that the work of grace in her is done. We believe that such a unique title could only hold completely true if she had been completed in grace since her conception and for all of her life. 

Now some of the supporting analogies from scripture:

The most famous analogical demonstration of Mary's freedom from sin is that she is the Ark of the New Covenant. In Exodus 25 it is said that the ark of the Old Covenant, which was but a shadow of the covenant to come, had to be made from pure gold  (verse 11). The ark contained manna, Aaaron's the priest's rod, and the inscribed words of the Ten Commandments. Mary contained the "True Bread from Heaven" (John 6:32), the Great High Priest (Heb 3:1), and the Word made Flesh (Jn 1:14). When God descended on the ark of the old covenant, it said that the Glory of God "overshadowed it". In the Septuagint (Greek) Old Testament this word was episkiasei In Luke Gabriel says ""The Holy Spirit will come upon you, and the power of the Most High will overshadow [episkiasei] you" The same word for overshadow in Greek is used as well. David the King leaped and danced before the Ark as it arrived in Jerusalem; compare John the Baptist who leaped in his mother's womb at Mary's arrival. If the Ark of the OLD Covenant was by necessity to be pure from blemish in a physical sense, wouldn't it be right and just that the Ark that held True God in the Flesh be free from spiritual blemish? Shouldn't her body, which contains the New and Definitive Covenant, and soul be free from every stain of Sin? And honestly, if you could give your mother any present at all free from cost, wouldn't you give her a life free from bondage of sin. We can't do that, but Jesus could. Jesus affection for his own mother, makes the gift of salvation from her birth reasonable in addition to it being proper to her as a vessel of the true and living presence of God.

The other analogical demonstation of Mary's sinlessness is that she is the New Eve. Both Adam and Eve were created without sin, but when faced with the Choice of God over self, they chose self. Eve's choice led to Adam's sin, leading to humanity's downfall. Mary was likewise presented with a choice that would lead to the Jesus Christ, the New Adam (see Romans 5:12-21) that would lead to humanity's salvation. To recapitulate humanity in Jesus, both the New Adam and the New Eve (Mary) must be without sin in order that her choice and his actions re-integrate humanity into relationship with God as it was meant to be from the beginning.