But having been made holy, we must preserve that holiness: we must live lives of obedience to God. This is what §11 of the Decree on Justification teaches us.
But no one, how much soever justified, ought to think himself exempt from the observance of the commandments; no one ought to make use of that rash saying, one prohibited by the Fathers under an anathema – that the observance of the commandments of God is impossible for one that is justified. For God commands not impossibilities, but, by commanding, both admonishes thee to do what thou are able, and to pray for what thou art not able (to do), and aids thee that thou mayest be able; whose commandments are not heavy; whose yoke is sweet and whose burthen light.
Understand: they are not saying that just anyone can do this, but rather that the man who is justified may do so. And he is able to do so not solely by his own strength, but by the grace of God which helps him to do so. This passage offers no help to those who seek a "works-based" salvation in the teaching of Trent (nor does any other passage, for that matter). The justified are the ones in view – those whom God has saved through Baptism, which (as we have seen) is entirely a work of grace.
That we must obey God is the unambiguous teaching of the New Testament, so that those who deny such a duty are in gross error.
For, whoso are the sons of God, love Christ; but they who love him, keep his commandments, as Himself testifies [John 14:15 – RdP]; which, assuredly, with the divine help, they can do. For, although, during this mortal life, men, how holy and just soever, at times fall into at least light and daily sins, which are also called venial, not therefore do they cease to be just. For that cry of the just, Forgive us our trespasses, is both humble and true. And for this cause, the just themselves ought to feel themselves the more obligated to walk in the way of justice, in that, being already freed from sins, but made servants of God, they are able, living soberly, justly, and godly, to proceed onwards through Jesus Christ, by whom they have had access unto this grace. For God forsakes not those who have been once justified by His grace, unless he be first forsaken by them. Wherefore, no one ought to flatter himself up with faith alone, fancying that by faith alone he is made an heir, and will obtain the inheritance, even though he suffer not with Christ, that so he may be also glorified with him. For even Christ Himself, as the Apostle saith, Whereas he was the son of God, learned obedience by the things which he suffered, and being consummated, he became, to all who obey him, the cause of eternal salvation. For which cause the same Apostle admonishes the justified, saying; Know you not that they that run in the race, all run indeed, but one receiveth the prize? So run that you may obtain. I therefore so run, not as at an uncertainty: I so fight, not as one beating the air, but I chastise my body, and bring it into subjection; lest perhaps, when I have preached to others, I myself should become a cast-away. So also the prince of the apostles, Peter; Labour the more that by good works you may make sure your calling and election. For doing those things, you shall not sin at any time. From which it is plain, that those are opposed to the orthodox doctrine of religion, who assert that the just man sins, venially at least, in every good work; or, which is yet more insupportable, that he merits eternal punishments; as also those who state, that the just sin in all their works, if, in those works, they, together with this aim principally that God may be gloried, have in view also the eternal reward, in order to excite their sloth, and to encourage themselves to run in the course: whereas it is written, I have inclined my heart to do all thy justifications for the reward: and, concerning Moses, the Apostle saith, that he looked unto the reward.
To deny our duty as Christians to obey God is to shred the New Testament. Faith without works is dead. There are Protestants who agree that we have a duty to obey God, and this is good; but their view is often undermined by a contradictory notion that the Christian cannot lose his salvation. This latter view is false, and gives the lie to such a man's idea that he has a duty: it's not a true duty if their are no consequences for failing to fulfill it. To the contrary, such a pairing of ideas reduces the "duty" to naught but a pious wish.