Original sin is passed from parent to child – it is inherited. Because of humanity’s fall, we are all conceived in this state which damages our relationship with God. Because of original sin, we are all in need of salvation. There is nothing in our power that we can do to overcome this fall from grace. Only an act of God can save us.
Mary was saved by an act of God. She was preserved from original sin so that she would be spotless, the perfect vessel for carrying God’s Son. The reason this was so necessary is quite simple. If Mary had possessed original sin, she would have passed it on to Jesus, her child. Jesus would inherit a fallen human nature. To prevent this, God could have “stepped in” at the moment of Jesus’ conception to save” Him from original sin. But can you imagine a Savior in need of salvation? …the Son of God in need of grace? This is obviously not a workable solution.
Instead Mary was spared so that Jesus would be born NATURALLY without the stain of original sin. It was not an act of Mary that caused this…she still owes her salvation to God, as we all do. But without this singular act of God’s grace the birth of the God-made-man would have been a theological impossibility. Mary was spared from original sin so that Jesus (who is God) could be born without need of salvation, so that he could offer himself unblemished as a sacrifice for us all.