But by anticipating the merits of Christ, God saved Mary before she fell in the mire of sin. As an example, a child can be saved from drowning after he falls into a swimming pool. He also could be saved from drowning by grabbing him before he falls in the pool. Our redemption is "healing medicine," but her redemption was like a "vaccine." She was always immune from sin, even venial sin (CCC 493). This is the most perfect kind of redemption.

The main Bible text that suggests this doctrine is the Angel Gabriel's greeting to Mary:

In the same passage, St. Paul writes that "none is righteous, no, not one" [Romans 3:10]. Yet elsewhere Noah, Daniel, Job (Ezek. 14:14,20), Zechariah and Elizabeth (Luke 1:6) are said to be righteous.

Romans 3:23, "all have sinned"

they were answered by Duns Scotus (1264–1308), who "developed the idea of preservative redemption as being a more perfect one: to have been preserved free from original sin was a greater grace than to be set free from sin".[30]