Augustine of Hippo, On the Merits and Forgiveness of Sins, and on the Baptism of Infants II.18: Perfect Human Righteousness is Imperfect

There are then on earth righteous men, there are great men, brave, prudent, chaste, patient, pious, merciful, who endure all kinds of temporal evil with an even mind for righteousness’ sake. If, however, there is truth—nay, because there is truth—in these words, “If we say we have no sin, we deceive ourselves,” and in these, “In Thy sight shall no man living be justified,” they are not without sin; nor is there one among them so proud and foolish as not to think that the Lord’s Prayer is needful to him, by reason of his manifold sins.