A good place to start would be with Chapter VIII of the same decree, titled "In what manner it is to be understood, that the impious is justified by faith, and gratuitously."
And whereas the Apostle saith, that man is justified by faith and freely, those words are to be understood in that sense which the perpetual consent of the Catholic Church hath held and expressed; to wit, that we are therefore said to be justified by faith, because faith is the beginning of human salvation, the foundation, and the root of all Justification; without which it is impossible to please God, and to come unto the fellowship of His sons: (source; emphasis added)The first thing we see is that Trent flatly affirms that there is a sense in which we may be said to be justified by faith! How can this be?!? We're told constantly that we deny any such thing! Well, no. What the Church rejects is the Protestant error on this matter.
But Chapter VIII says more.
but we are therefore said to be justified freely, because that none of those things which precede justification-whether faith or works-merit the grace itself of justification. For, if it be a grace, it is not now by works, otherwise, as the same Apostle says, grace is no more grace (ibid.)Why is it said that we are justified freely? Because nothing that precedes that justification in any way merits it. It is sheer grace.
So what about our merits? They come from Christ.
For, whereas Jesus Christ Himself continually infuses his virtue into the said justified,-as the head into the members, and the vine into the branches,-and this virtue always precedes and accompanies and follows their good works, which without it could not in any wise be pleasing and meritorious before God... (ibid., ch. 16)See? Any good works we do are preceded by grace, and accompanied by grace, and followed by grace, and apart from grace "could not in any wise be pleasing and meritorious before God".
But there is more.
Thus, neither is our own justice established as our own as from ourselves; nor is the justice of God ignored or repudiated: for that justice which is called ours, because that we are justified from its being inherent in us, that same is (the justice) of God, because that it is infused into us of God, through the merit of Christ (ibid; emphasis added).Again: no merit on our part except that merit which we have from Christ. And this is consistent with what St. Augustine says (Letter 194).
But, have the just no merits at all? Certainly they have, since they are just; only, there were no previous merits to make them just. They became just when they were justified, but, as the Apostle says, 'They are justified freely by his grace.' ...So: just as Trent says, St. Augustine says also. We have merits (after we have been justified) - and God rewards them - but these merits are themselves gifts from God. So there is no room for boasting. It is grace, from start to finish. Anyone who says otherwise just doesn't understand what we believe.
What merit, then, has man before grace which could make it possible for him to receive grace, when nothing but grace produces good merit in us; and what else but His gifts does God crown when He crowns our merits? (emphasis added)