Dear friends on Catholic Answers Forums: I am a prospective converts to Catholicism and am trying to learn some Catholic Mariology. Could someone please answer/explain this question with necessary references to authoritative documents?
Here’s the entire papal document that defines the Immaculate Conception infallibly. I recommend reading the whole thing as it gives some reasons from Scripture and history for why the Church has always believed this teaching.
papalencyclicals.net/Pius09/p9ineff.htm
As for your question, here’s the relevant quote from the document:
“We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception,
by a singular grace and privilege granted by Almighty God,
in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.”
I bolded the parts that answer your question. Mary was given a unique privilege that no other human being (besides Adam and Eve and Christ obviously) has or will be granted. And it was all in view of Christ’s sacrifice. Meaning Mary WAS saved by Christ’s death on the cross, but since God is outside of time, he was able to apply those graces to Mary before Christ had actually died. (Little time-paradox there for you

) But Jesus’ sacrifice was still necessary for Mary, just like it is for us.
If you want to read more, I recommend the paragraphs in the Catechism of the Catholic Church that deal with Mary and her role in salvation history. Here’s a link to some of them:
scborromeo.org/ccc/p122a3p2.htm#484
Hope that helps! God bless you in your journey.