Well, here's what I wrote to my friend in explanation. An explantion, by the way, he received very well.
Christ was conceived in a Virgin – Mary was conceived the ol’ fashioned way.
Christ was conceived Divine and sinless - Mary was conceived sinless.
Christ does not need a Savior – Mary did need a Savior.
As echoed in her Canticle in Luke when she rejoices “in God her Savior.” The only difference rests in how and why that salvation was given in the way it was. Being human, she certainly would have caught the stain of Adam’s sin, if it weren’t for her being saved in anticipation of the merits of Christ on Calvary, like us, but before us. This was done precisely because it was Mary who was chosen to carry God in her womb which can call for some purifying sanctification. Mary’s parents conceived a sinless human, while Mary herself conceived the sinless God. This is why the objection of an infinite regression is mute – Mary could be without sin, without her parents and their parents and their parents ad nauseum.
This was not a mid-19th century “Catholic invention.’
Though the issues were laid out and clearly defined by Pope Pius IX in 1854 in the encyclical Ineffabilis Deus, her sinlessness was arguably as strongly held as the Trinity from the earliest disciples to even Martin Luther and even the first Protestants. There was, however, a difference in timing: some believed she was conceived without sin, while the rest believed her freedom from it came when she was born. For example, St. Thomas Aquinas believed it came at her birth, while St. Bonaventure believed it was at conception and so on. No one (to my knowledge) in the early church never spelled out when it happened so much as they agreed that it did.
You’ve heard of “Hail Mary, full of grace.” This is the literal translation of Luke 1:28. 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. Not by her own merits, or because she was just better than the rest of us on some exalted throne, but because for whatever reason, God chose this woman to bear Himself to the world. It could be argued that having His mother be a pure vessel wasn’t 100% necessary, but like the Ark of the Covenant made from the purest gold and carrying the Word of God inside, God in His desire chose to make Mary pure so the Word of God she carried would have the purest of Incarnations.
But wait, ye may say, what about Romans 3:23 which states that all have sinned and fallen short of the glory of God? When reading through Paul’s discourse we see not that he is speaking of specific individuals, but rather of tribes or groups of people: Jew, Greek, man, woman, slave, free ect. He begins the talk asking “what advantage has one who is a Jew.” So when he says “all” have sinned, the argument can be made that he’s referencing that all of humanity’s classes have fallen, but not necessarily all men. Case in point, what about babies or the unborn? What about Jesus himself? Contradictions would abound, but that’s why the context is not individually based, but rather collectively based.
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