22 After this we also entreat for our sins, saying, “And forgive us our debts, as we also forgive our debtors.” After the supply of food, pardon of sin is also asked for, that he who is fed by God may live in God, and that not only the present and temporal life may be provided for, but the eternal also, to which we may come if our sins are forgiven; and these the Lord calls debts, as He says in His Gospel, “I forgave you all that debt, because you desired me.” And how necessarily, how providently and salutarily, are we admonished that we are sinners, since we are compelled to entreat for our sins, and while pardon is asked for from God, the soul recalls its own consciousness of sin!
Lest any one should flatter himself that he is innocent, and by exalting himself should more deeply perish, he is instructed and taught that he sins daily, in that he is bidden to entreat daily for his sins. Thus, moreover, John also in his epistle warns us, and says, “If we say that we have no sin, we deceive ourselves, and the truth is not in us; but if we confess our sins, the Lord is faithful and just to forgive us our sins.” In his epistle he has combined both, that we should entreat for our sins, and that we should obtain pardon when we ask. Therefore he said that the Lord was faithful to forgive sins, keeping the faith of His promise; because He who taught us to pray for our debts and sins, has promised that His fatherly mercy and pardon shall follow.
Source: The Treatises of Cyprian (New Advent)