From an old email to an Anglican friend of decidedly Calvinist leanings...
"Two big issues there, but I’ll do my best with one anyway. For the sake of brevity I’ll put off saints in heaven to another day, as I need to think about how to explain that concisely. It takes time to write short emails.
Mary’s preservation from sin has been mainstream Christian belief from about as early as we can tell in Christian history; it’s been embraced through history by Catholics, the Orthodox churches, and such Protestants as Martin Luther. Partly, it has to be said, the belief derives from incredulity that the flesh from which Jesus was made – Mary’s and Mary’s alone – could ever have been tainted by sin and been what C.S. Lewis called ‘enemy-occupied territory’.
At Luke 1:28, the angel salutes Mary as Kekharitōmenē, traditionally translated ‘Full of Grace’, but more accurately rendered as something along the lines of ‘you who are already, absolutely, and enduringly endowed with grace’. It’s an extraordinarily unusual grammatical form, and is also the only instance we know of that an angel ever honoured any of us with a title. Being completely graced by God leaves no room for sin; this can’t be dismissed as though it just means ‘highly favoured’.
Following 1 Corinthians 15:45-9, the early Church looked at Genesis 3:15 and saw our redemption as a re-enactment of Eden: just as Eve’s disobedience opened the way to Adam’s sin, so Mary’s obedience opened the way to Jesus’ saving of us. You’ll see this in such second-century writers as Justin Martyr and Irenaeus of Lyons, who, as you’ll know, was taught by one of John’s pupils and was the first person we know of who to describe the four Gospels – and those alone – as canonical. By the fourth century, it was almost proverbial that Mary was the new Eve, and could hardly have been created less than her: as Eve was created sinless, so must Mary have been.
Jude 1:24-5 points to how this could have happened in a way in harmony with the rest of the Bible. There are two ways of saving people: you can rescue them after they’ve fallen into a hole, or you can prevent them from falling in to begin with. It’s obviously better to be saved by prevention than cure, and it’s in this sense Mary was saved by Jesus: he preserved her from sin, rather than allowing her to fall into it and dragging her from the hole like the rest of us. And why wouldn’t he? He loved and honoured his mother: if he could save her by preserving her from sin, rather than allowing her to fall into sin in the first place, why would he have done otherwise?
The point of the Immaculate Conception is to glorify Jesus, the new Adam made from untainted flesh and the perfect saviour who is able to keep us from falling."
His reply, to my amazement, suggested I'd almost convinced him and that he needed to think more on this. I must ask where he got to with that.