15 Our Lord so instructed His Apostles that, daily at the sacrifice of His body, believers make bold to say, “Our Father, Which art in Heaven, hallowed be Your name”; they earnestly desire the name of God, which in itself is holy, to be hallowed in themselves; you say, “You know, Lord, how holy, how innocent, and how pure are my hands.” Then they say: “Your Kingdom come,” anticipating the hope of the future kingdom, so that, when Christ reigns, sin may by no means reign in their mortal body, and to this they couple the words, “Your will be done in earth as it is in Heaven”; so that human weakness may imitate the angels, and the will of our Lord may be fulfilled on earth; you say, “A man can, if he chooses, be free from all sin.”
The Apostles prayed for the daily bread, or the bread better than all food, which was to come, so that they might be worthy to receive the body of Christ; and you are led by your excess of holiness and well established righteousness to boldly claim the heavenly gifts. Next comes, “Forgive us our debts, as we also forgive our debtors.” No sooner do they rise from the baptismal font, and by being born again and incorporated into our Lord and Saviour thus fulfil what is written of them, “Blessed are they whose iniquities are forgiven and whose sins are covered,” than at the first communion of the body of Christ they say, “Forgive us our debts,” though these debts had been forgiven them at their confession of Christ; but you in your arrogant pride boast of the cleanness of your holy hands and of the purity of your speech.
However thorough the conversion of a man may be, and however perfect his possession of virtue after a time of sins and failings, can such persons be as free from fault as they who are just leaving the font of Christ? And yet these latter are commanded to say, “Forgive us our debts, as we also forgive our debtors”; not in the spirit of a false humility, but because they are afraid of human frailty and dread their own conscience. They say, “Lead us not into temptation”; you and Jovinian unite in saying that those who with a full faith have been baptized cannot be further tempted or sin.
Lastly, they add, “But deliver us from the evil one.” Why do they beg from the Lord what they have already by the power of free will? Oh, man, now you have been made clean in the laver, and of you it is said, “Who is this that comes up all white, leaning upon her beloved?” The bride, therefore, is washed, yet she cannot keep her purity, unless she be supported by the Lord. How is it that you long to be set free by the mercy of God, you who but a little while ago were released from your sins? The only explanation is the principle by which we maintain that, when we have done all, we must confess we are unprofitable.
Source: Against the Pelagians (New Advent)