48 What now shall I say concerning the very carefulness and watchfulness against sin? “Who shall boast that he has a chaste heart? Or who shall boast that he is clean from sin?” Holy virginity is indeed inviolate from the mother's womb; but “no one,” says he, “is clean in Your sight, not even the infant whose life is of one day upon the earth.” There is kept also in faith inviolate a certain virginal chastity, whereby the Church is joined as a chaste virgin unto One Husband: but That One Husband has taught, not only the faithful who are virgin in mind and body, but all Christians altogether, from spiritual even unto carnal, from Apostles even unto the last penitents, as though from the height of heaven even unto the bounds of it, to pray, and in the prayer itself has admonished them to say, “And forgive us our debts, even as we also forgive our debtors:” where, by this which we seek, He shows what also we should remember that we are.
For neither on behalf of those debts, which for our whole past life we trust have been forgiven unto us in Baptism through His peace, has He charged us to pray, saying, “And forgive us our debts, even as we also forgive our debtors:” otherwise this were a prayer which Catechumens rather ought to pray up to the time of Baptism; but whereas it is what baptized persons pray, rulers and people, pastors and flocks; it is sufficiently shown that in this life, the whole of which is a trial, no one ought to boast himself as though free from all sins.
Source: On Holy Virginity (New Advent)