Here is an argument, mostly directed toward Protestant Christians, in favor of the Catholic doctrine of the sinlessness of Mary.[1]
1. Grace is that by which God applies the Saving power of Jesus's sacrifice. (Scriptural supports include Eph. 2:8, "For by Grace you have been saved..." and Rom. 6:14, "sin will have no dominion over you, since you are ...under Grace.")
2. Mary was "full of Grace" [kecharitomene]. (According to the declaration of the angel Gabriel, as recorded in Lk. 1:28.)
3. Therefore, Mary was full of that by which God applies the Saving power of Jesus's sacrifice. (From 1 and 2.)
4. To be "full" of something is to be unable to accommodate any more of whatever that something is. (This just seems to me to be the primary meaning of the word "full.")
5. Therefore, Mary's being "full of Grace" is for her to have been unable to accommodate any more Grace. (From 2 and 4.)
6. If she had lacked Grace at any time, then she would have been able to accommodate more Grace. (Admittedly, this is the least obvious premise. However, I think that it is plausible. Although, with something like water, a bucket's being full-today does not prevent it's having been empty yesterday, in the case of God's Grace, things are arguably different. God is able to view and consider our lives in toto - from their beginnings. It is reasonable to think that if Mary had lacked Grace at any time, then she wouldn't really have been "full" with it.)
7. Therefore, Mary did not lack grace at any time. (From 5 and 6 by modus tollens.)
8. Therefore, Mary did not lack the the Saving power of Jesus's sacrifice at any time. (By 1 and 7.)
9. If Mary did not, at any time, lack the Saving power of Jesus's sacrifice, then Mary was never separated from God by sin.
10. Therefore, Mary was never separated from God by sin. (From 8 and 9, by modus ponens.)
11. Any person never separated from God by sin is sinless. (From the definition of the "sin.")
12. Therefore, Mary is sinless.
Here is an objection.
If it was requisite that Jesus's parents, God the Father and Mary, be without sin, then it should be requisite that Mary's parents, Joachim and Ann, be without sin.
One way of cashing this out would be as follows. Call this Argument A.
13. If a person, S, is born without sin, then S's parents are sinless.
14. Mary was born without sin.
15. Therefore, Mary's parents were sinless.
There is a confusion, here, rooted in an ambiguity contained in the idea of sinlessness.
On the one hand, someone might think that the Catholic claim is that Mary's freedom from Original Sin was not owed to anything but to her own Good nature. Let us call this the idea that Mary was "necessarily free" from Original Sin.
As far as I can tell, this is not, nor has it ever been, the Catholic claim.
For on the other hand, Catholics answer that Mary's freedom from Original Sin was owed to God's Grace. On this view, the Catholic view, Mary was "contingently free" from Original Sin.
This is no mere verbal jousting. If the claim were indeed that Mary owed God nothing in virtue of her freedom from Original Sin, then the notion that Mary "had no need for a savior" would be obviously true.
On the actual claim, Mary owed her preservation from Original Sin to God. Thus, Mary certainly did need - and had - a savior.
At least one reason to think that Argument A is a failure can be gleaned from this distinction. A person, S, is "necessarily-sinless" if S's sinlessness is essential and could not be otherwise. Moreover, if S *is* necessarily-sinless, then S does not need a savior. If, on the other hand, S is "contingently-sinless," then S's sinlessness could have been otherwise.
Taking these distinctions into account, we could formulate Argument B:
16. If a person, S, is necessarily born without sin, then S's parents are sinless.
17. Jesus was necessarily born without sin.
18. Therefore, Jesus's parents (Mary and God the Father) were sinless.
But Argument B is not extended to Mary. Consider Argument C:
16. If a person, S, is necessarily born without sin, then S's parents are sinless.
19. Mary was necessarily born without sin.
20. Therefore, Mary's parents (Joachim and Ann) were sinless.
Argument C is unsound - by Catholic lights - since premise 7. is false.
It is not the case that "Mary was necessarily born without sin." Mary was contingently born without sin. Her sinlessness was entirely at the Grace and pleasure of God. Jesus's sinlessness was essential. As the Second Person of the Blessed Trinity, his paternity conferred sinlessness to Him. But since, again by Catholic lights, Mary was contingently-sinless, Jesus's maternity also conferred sinlessness to Him.
Hence, Jesus was necessarily-sinless because both of his parents were sinless. His thus being essentially sinless secured His status as savior who did not need saving Himself.
Still, Mary's sinlessness arguably, partially secured Jesus's sinlessness. Although, as has been stated, Mary's sinlessness was neither due to her own nature nor to anything that she had accomplished. Mary was, in other words, saved as the rest of us are: "...by grace ...through faith - and this is not from [herself], it [was] the gift of God - not by works, so that [she cannot] boast". (Ephesians 2:8-9, NIV.)
And, indeed, "boastfulness" is nearly as far from the character of the Virgin Mary as could be conceived. This is why Mary exclaimed: "[M]y spirit rejoices in God my Savior" (Luke 1:47, NIV), and why, henceforth, "all generations will call [Mary] blessed" (Luke 1:48, NIV.)
There is no impropriety in reporting that Mary was saved by God's Grace. It is simply that in her case, the saving Grace was given to her at conception, preserving her from the stain of Original Sin, whereas in our cases, the grace comes after our births.
But there is another objection.
Some Protestants immediately object on the ground that verses such as Romans 3:23 and Romans 5:12 (and so on) imply that Mary sinned.
The problem, here, is straightforward.
21. All [humans] have sinned. (Romans 3:23.)
22. Mary was human. (Self-evident.)
23. Therefore, Mary sinned. (From 21 and 22.)
To see at least one problem with this, consider a parallel argument.
21. All [humans] have sinned. (Romans 3:23.)
24. Jesus was fully human. (From Christian theology.)
25. Therefore, Jesus sinned. (From 9 and 10.)
Obviously, 25. is unacceptable to any Protestant. What can be said about this? What could a Protestant say?
One first-pass reply surely would be for the Protestant to point out that we have good reason, from other Bible passages in the Bible (such as 2 Corinthians 5:21, Hebrews 4:15, etc.) to think that the word "all" in passages like Romans 3:23 (et alia) really does not mean "each and every human without exception."
A principle might be asserted such that: "All" means "each and every, unless we have good Scriptural grounds thinking it does not." In the case of Jesus, the Protestant would surely hold, we have good Scriptural grounds for thinking that it does not.
Actually, this need not be construed as making an "exception." One way to view it is in terms of what philosophers call a "restricted domain." A common example goes like this. If I tell John: "Put all the beer into the refrigerator," I probably should not best be understood as telling John to get all of the beer that there is. Probably, I mean something more like all of the beer that I have in the shopping bags, or all of the beer that there is on my table. The point is, most likely, I am using the quantifier "all" in a restricted sense.
Surely it is plausible to think that Saint Paul is using the word "all" similarly in Romans. On this reading, Paul means (something like) "For all of my readers have sinned."
Of course, we could also hold that Paul did indeed imply an exception. Whereas Protestants may believe that Paul meant "For all have sinned except Jesus," Catholics may hold that Paul meant "For all have sinned except the New Adam [Jesus] and the New Eve [Mary]."
Protestants justify their reading by appealing to Scriptural counter-evidence. Catholics simply believe that, in addition to having good ground to make an exception for Jesus, we also have good reason make an exception for Mary.
This does not place Mary on the "same level" as Jesus. As I argued previously, Jesus's sinlessness was essential to him, whereas Mary's was (on the Catholic view) contingent. Jesus was the savior. Mary was saved by Grace (it's just that she was so saved from the moment of her birth).
Finally, Protestants might say: "Yes, but in Jesus's case, the New Testament is more explicit about this exception; Mary's 'exceptional' status must be inferred."
As far as I can tell, to address this worry, Catholics merely need to do two things. Number one, they need to provide a good reason why the the New Testament is not more explicit about Mary's sinlessness. This does not seem too difficult to do. There appears to be a fairly straightforward reason close at hand. The New Testament is primarily about Jesus (in the Gospels) and about Jesus's Church (from Acts onward). Therefore, we receive the most information in the New Testament regarding Jesus and the Church.
However, as a Catholic, I do not believe that the New Testament is entirely silent about Mary's sinlessness. I believe that the New Testament's clear references to Mary as the "highly favored daughter" who is "full of Grace" and who will, by all future Christians, be called "blessed" are best-explained by the doctrine that Mary was sinless.
This leads to number two, Catholics must provide a good reason to think that Mary was sinless. But this good reason has already been set forth. It can be coherently argued from the Biblical datum of Luke 1:28 that Mary's having been "full of Grace" implies her sinlessness.
In order to rebut this, Protestants must find some fault with the argument that was given in its favor. Short of this, Catholics seem to me to be quite within their rational rights to hold that Mary was indeed sinless, as the Church has believed since ancient times.
Notes:
[1] This argument is not due to me, but has been adapted and expanded by me from David Armstrong, The Catholic Verses, Manchester, N.H.: Sophia Inst. Press, 2004, pp. 181ff.