is not remission of sins merely, but also the sanctification and renewal of the inward man, through the voluntary reception of the grace, and of the gifts, whereby man of unjust becomes just, and of an enemy a friend, that so he may be an heir according to hope of life everlasting.
So justification is not merely a forensic act but is forgiveness of sins and sanctification: we become literally holy. This work is accomplished in us "through the voluntary reception of the grace and of the gifts" of God; hence it is not in us of ourselves. We for our part must freely accept this grace freely offered.
More important than this definition – at least for our purposes in this series – is the rest of the chapter, which deals with the causes of justification. Trent tells us that our sins remitted, that we are sanctified and renewed in the inner man, that we become just, that we become friends of God, that we become heirs of life everlasting; but what causes all this?
[T]he final cause indeed is the glory of God and of Jesus Christ, and life everlasting
Final causes are the reasons why things are done – the purpose in doing them. Trent tells us that God's purpose in justifying us is his own glory, and eternal life for us. Though it's true that it's not explicitly said that the final cause here is God's, who else can rationally be in view? As we see below, the efficient cause of our salvation is God himself; it would be irrational (and heretical) to suppose that anything could move God to act.
the efficient cause is a merciful God who washes and sanctifies gratuitously, signing, and anointing with the holy Spirit of promise, who is the pledge of our inheritance
Efficient causes are those things which actually bring about a result or effect. In this case we see that the efficient cause is God himself: in other words, he justifies us when he graciously washes and sanctifies us. If (as some falsely allege) the Catholic Church taught a "works-based" salvation, would we not see it here? And yet we do not. The Church teaches no such thing; rather, as we see, she teaches that it is God who justifies us, not we ourselves.
the meritorious cause is His most beloved only-begotten, our Lord Jesus Christ, who, when we were enemies, for the exceeding charity wherewith he loved us, merited Justification for us by His most holy Passion on the wood of the cross, and made satisfaction for us unto God the Father
A meritorious cause isn't actually one of Aristotle's four causes, but there is still a sense in which we can rightly speak of such a thing: a meritorious cause could be described as that which brings about a result by reason of merit, so that the effect is deserved because the merits of the cause. If, as some falsely allege, the Catholic Church taught that we can merit our own salvation in some absolute sense, we would expect to see that reflected here, right? And yet we don't – again, precisely because that is not what the Church teaches. Rather, the merit that warrants our justification belongs to Jesus Christ alone.
the instrumental cause is the sacrament of baptism, which is the sacrament of faith, without which (faith) no man was ever justified
The means by which God applies this salvation to us is the instrument of Holy Baptism.
[T]he alone formal cause [again, see here for a brief description – RdP] is the justice of God, not that whereby He Himself is just, but that whereby He maketh us just, that, to wit, with which we being endowed by Him, are renewed in the spirit of our mind, and we are not only reputed, but are truly called, and are, just, receiving justice within us, each one according to his own measure, which the Holy Ghost distributes to every one as He wills, and according to each one's proper disposition and co-operation.
The formal cause is God's justice whereby he makes us just.
That's all, folks. Trent declares no other causes of our salvation besides these. How then can it be said that we believe in a "works-based" justification? Well, it can't. Unfortunately, however, there are many silly people who foolishly say otherwise, ignoring or ignorant of the facts we have just reviewed.
Some of these folks may look at the rest of §7 and draw bad conclusions.
For, although no one can be just, but he to whom the merits of the Passion of our Lord Jesus Christ are communicated, yet is this done in the said justification of the impious, when by the merit of that same most holy Passion, the charity of God is poured forth, by the Holy Spirit, in the hearts of those that are justified, and is inherent therein: whence, man, through Jesus Christ, in whom he is ingrafted, receives, in the said justification, together with the remission of sins, all these (gifts) infused at once, faith, hope, and charity. For faith, unless hope and charity be added thereto, neither unites man perfectly with Christ, nor makes him a living member of His body. For which reason it is most truly said, that Faith without works is dead and profitless; and, In Christ Jesus neither circumcision, availeth anything, nor uncircumcision, but faith which worketh by charity. This faith, Catechumen's beg of the Church-agreeably to a tradition of the apostles-previously to the sacrament of Baptism; when they beg for the faith which bestows life everlasting, which, without hope and charity, faith cannot bestow: whence also do they immediately hear that word of Christ; If thou wilt enter into life, keep the commandments. Wherefore, when receiving true and Christian justice, they are bidden, immediately on being born again, to preserve it pure and spotless, as the first robe given them through Jesus Christ in lieu of that which Adam, by his disobedience, lost for himself and for us, that so they may bear it before the judgment-seat of our Lord Jesus Christ, and may have life everlasting. [emphasis added]
The simple fact is that although God saves us, we most certainly can reject that salvation and break fellowship with him by sinning mortally. May God preserve us from ever doing so. Consequently we have a duty – one we ought to fulfill out of grateful love for our gracious Savior – to obey God, and to keep his commandments (Jn. 14:15).