De natura et gratia libri III, cum Apologia contra Catharinum

But let no one, however justified, think himself free from the observance of the commandments. Let no one use that rash expression, forbidden by the Fathers under anathema, that the commandments of God are impossible for a justified man to observe. For God does not command what is impossible, but by commanding he warns us both to do what you can and to ask for what you cannot, and he helps you so that you can, whose commandments are not burdensome, whose yoke is sweet and whose burden is light. For those who are sons of God love Christ; but those who love him (as he himself testifies) keep his words. This indeed, with divine help, they can accomplish. For although in this mortal life, however holy and just they are, they sometimes fall into light and daily sins, which are also called venial, they do not therefore cease to be just. For that voice of the just is both humble and true: Forgive us our debts. Whence it comes about that the just themselves ought to feel themselves the more bound to walk in the way of justice, the more, now freed from sin and made servants of God, living soberly, justly and piously, they can perfect it through Christ Jesus, through whom they have obtained access to this grace. For God does not abandon those once justified by his grace, unless he is first abandoned by them. And so no one ought to flatter himself in faith alone, thinking that by faith alone he has been made an heir and will obtain the inheritance, even if he does not suffer with Christ so as also to be glorified with him. For Christ himself also (as the Apostle says), although he was the Son of God, learned obedience from the things he suffered; and being made perfect, he became the cause of eternal salvation to all who obey him. Therefore the Apostle himself warns the justified, saying: Do you not know that those who run in the stadium all indeed run, but one receives the prize? So run that you may obtain it. I therefore so run, not as if into uncertainty; I so fight, not as if beating the air; but I chastise my body and bring it into subjection, lest perhaps, when I have preached to others, I myself should be rejected. Likewise the prince of the Apostles Peter: Be zealous to make your calling and election certain through good works. For by doing these you will never sin. Hence it is clear that those who say that the just man sins at least venially in every good work, or (what is more intolerable) that he deserves eternal punishments, are opposed to the doctrine of the orthodox religion. And so too are those who hold that the just sin in all their works, if in those works, by stirring up their own sloth and exhorting themselves to run in the stadium, they also look to an eternal reward, so that God may be glorified above all—since it is written, I have inclined my heart to do your justifications for the sake of recompense, and the Apostle says of Moses that he was looking to the reward.