Scripture doesn’t say Mary was. It says she was “favored”. It does say Stephen was “ full of grace” but not sinless.
Irenaeus
Consequently, then, Mary the Virgin is found to be obedient, saying, “Behold, 0 Lord, your handmaid; be it done to me according to your word.” Eve . . . who was then still a virgin although she had Adam for a husband — for in paradise they were both naked but were not ashamed; for, having been created only a short time, they had no understanding of the procreation of children . . . having become disobedient [sin], was made the cause of death for herself and for the whole human race; so also Mary, betrothed to a man but nevertheless still a virgin, being obedient [no sin], was made the cause of salvation for herself and for the whole human race. . . . Thus, the knot of Eve’s disobedience was loosed by the obedience of Mary. What the virgin Eve had bound in unbelief, the Virgin Mary loosed through faith (Against Heresies 3:22:24 [A.D. 189]).
Augustine
We must except the Holy Virgin Mary, concerning whom I wish to raise no question when it touches the subject of sins, out of honor to the Lord; for from Him we know what abundance of grace for overcoming sin in every particular was conferred upon her who had the merit to conceive and bear Him who undoubtedly had no sin (Nature and Grace 36:42 [A.D. 415]).
“It is a sweet and pious belief that the infusion of Mary’s soul was effected without original sin; so that in the very infusion of her soul she was also purified from original sin and adorned with God’s gifts, receiving a pure soul infused by God; thus from the first moment she began to live she was free from all sin.”
Martin Luther, (Sermon: “On the Day of the Conception of the Mother of God,” 1527).
Scripture doesn’t say Mary was. It says she was “favored”. It does say Stephen was “ full of grace” but not sinless.