In opposition also to the subtle wits of certain men, who, by pleasing speeches and good words, seduce the hearts of the innocent, it is to be maintained, that the received grace of Justification is lost, not only by infidelity whereby even faith itself is lost, but also by any other mortal sin whatever, though faith be not lost; thus defending the doctrine of the divine law, which excludes from the kingdom of God not only the unbelieving, but the faithful also (who are) fornicators, adulterers, effeminate, liers with mankind, thieves, covetous, drunkards, railers, extortioners, and all others who commit deadly sins; from which, with the help of divine grace, they can refrain, and on account of which they are separated from the grace of Christ.
To be sure, one may lose the grace of justification if he loses faith; but this is not the only sin by which one may lose salvation. The Fathers of Trent appeal to Galatians 5 here.
I say then: Walk in the spirit: and you shall not fulfill the lusts of the flesh. For the flesh lusteth against the spirit: and the spirit against the flesh: For these are contrary one to another: so that you do not the things that you would. But if you are led by the spirit, you are not under the law. Now the works of the flesh are manifest: which are fornication, uncleanness, immodesty, luxury, Idolatry, witchcrafts, enmities, contentions, emulations, wraths, quarrels, dissensions, sects, Envies, murders, drunkenness, revellings, and such like. Of the which I foretell you, as I have foretold to you, that they who do such things shall not obtain the kingdom of God.
The Christian may not live however he pleases. He is a servant of Christ, and if he loves Christ, he will obey him (John 14:15). But if we do not obey him in such things as St. Paul enumerates in Galatians, then we betray the fact that we do not really love him. As we've seen, though, by God's grace we may avoid such sins, and by his grace we may be restored if we do stumble into them.