From today's daily Mass reading:
The angel Gabriel was sent from God
to a town of Galilee called Nazareth,
to a virgin betrothed to a man named Joseph,
of the house of David,
and the virgin’s name was Mary.
And coming to her, he said,
“Hail, full of grace! The Lord is with you.”
Today celebrates the conception of Mary without sin. The Immaculate Conception is one of the very few doctrines in modern times that have been proclaimed infallibly by a pope, teaching ex cathedra. Perhaps even more understandably confusing than papal infallibility, though, is the dogma of the Immaculate Conception itself.
Quick notes on the Immaculate Conception:
1. It refers to the conception of Mary without original sin: from the first, she was "full of grace." Does not refer to the Incarnation: the conception of Christ in the womb of Mary by the power of the Holy Spirit.
2. It doesn't mean that Mary didn't need to be saved from sin by Christ's sacrifice on the Cross. It's just that in Mary's case, Christ's redeeming merit on the Cross worked backward in time to preserve her from original sin before she was conceived, and before He was incarnated.
3. God is eternal (i.e., trans-temporal), so while it's weird to contemplate the effects of Christ's sacrifice back-propagating through time, it's not illogical. Heck, on a Feynman diagram, even ordinary photons exhibit retrocausality. (Not that the physics explains the miracle or vice versa--it's just an analogy: Let's not get all Dancing Wu Li Masters here.)
4. Christ's atoning sacrifice has saved other people who predate His Incarnation, too--if in less spectacular fashion: Abraham, Moses, and the prophets, e.g., are in Heaven.
5. Mary is often referred to as the new Eve: Eve, also born without sin, said "No" to God's will in the Garden, but when the angel Gabriel told Mary of God's will, Mary said a complete "Yes." Given the same opportunity to make a radically free choice, the Blessed Mother of us all succeeded where the earthly mother of us all had failed. Mary is mankind's magnificent mulligan.
6. Mary is also often referred to as the Ark of the New Covenant. Just as the Ark where God dwelt among His Chosen People at the time of the Old Covenant was ritually pure, so the Ark where the Word became flesh was without sin.
But hey, this is just a blog. Obviously, I'm not even beginning to do this topic justice. Or anything close. Here's somewhere to learn more.