Augustine’s view on Virgin Mary
Pelagius (c. 350 to 425 AD) was founder of movement that emphasizes free-will and good works. The so-called Pelagian controversy was initiated by one of his followers, Celestius, whose teachings were condemned, among which the one that says even before the coming of Christ there were persons who lived without sin. In response to this Pelagians teaching, Augustine (c. 354 to 430 AD) replied that all of them were sinners, with the exception of Virgin Mary.
We must except the holy Virgin Mary, concerning whom I wish to raise no question when it touches the subject of sins, out of honour 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. Well, then, if, with this exception of the Virgin, we could only assemble together all the fore-mentioned holy men and women, and ask them whether they lived without sin whilst they were in this life, what can we suppose would be their answer? Would it be in the language of our author, or in the words of the Apostle John? I put it to you, whether, on having such a question submitted to them, however excellent might have been their sanctity in this body, they would not have exclaimed with one voice: “If we say we have no sin, we deceive ourselves, and the truth is not in us?”[1 John 1:8] But perhaps this their answer would have been more humble than true!
Augustine, On Nature and Grace 42 (36)
Nicene and Post Nicene Fathers, 1st Series, Vol. 5, page 135