But, though He died for all, yet do not all receive the benefit of His death, but those only unto whom the merit of His passion is communicated.
Continuing from where they left off in §2 (where they quoted 1 John 2:2), they clarify that although Christ is indeed the propitiation for our sins and those of the world, yet not all are blessed thereby. They then proceed to draw that same analogy made by St. Paul in Romans 5: "For if by one man's offence death reigned through one; much more they who receive abundance of grace and of the gift and of justice shall reign in life through one, Jesus Christ (Rom. 5:17).
For as in truth men, if they were not born propagated of the seed of Adam, would not be born unjust, – seeing that, by that propagation, they contract through him, when they are conceived, injustice as their own, – so, if they were not born again in Christ, they never would be justified; seeing that, in that new birth, there is bestowed upon them, through the merit of His passion, the grace whereby they are made just.
[which new birth, as we shall see later, is received through Baptism]
We also see here that without this new birth we would not be justified, because in it we receive the grace by which we are made just. Indeed, that is their very point: without that new birth, men would never be justified, "seeing that" – in other words, because – in that new birth God bestows ("through the merit of [Christ's] passion") "the grace whereby they are made just."
It is grace which makes us just - which justifies us. Consequently they are flatly mistaken who say that Trent teaches otherwise, and I can only assume that they have either never read it at all, or are only familiar with pull-quotes ripped by others out of the context of the Decree as a whole.
What is more, Trent would not call us to give thanks to God for our justification if it was a result of our own works.
For this benefit the apostle exhorts us, evermore to give thanks to the Father, who hath made us worthy to be partakers of the lot of the saints in light, and hath delivered us from the power of darkness, and hath translated us into the Kingdom of the Son of his love, in whom we have redemption, and remission of sins.
We give thanks to the Father precisely because he has done these things for us through Christ – things which include our redemption and the remission of sins.