Whence it came to pass, that the heavenly Father, the father of mercies and the God of all comfort, when that blessed fulness of the time was come, sent unto men, Jesus Christ, His own Son – who had been, both before the Law, and during the time of the Law, to many of the holy fathers announced and promised – that He might both redeem the Jews who were under the Law, and that the Gentiles, who followed not after justice, might attain to justice, and that all men might receive the adoption of sons. Him God hath proposed as a propitiator, through faith in his blood, for our sins, and not for our sins only, but also for those of the whole world. [§2]
"Whence it came to pass…": This chapter continues the thought begun in §1, discussed here. The sense, then, is: "Since it is the case that man caught in sin cannot justify himself either by nature or by deeds…" This being the case, God sent his Son Jesus Christ. He takes action on our behalf, it being impossible for us to climb out of the hole we have dug for ourselves.
So Christ came to redeem the Jews, and so that we Gentiles "might attain to justice," and so that we all (Jews and Gentiles) might receive adoption as sons. Secondly, he came to be "a propitiator, through faith in his blood, for our sins, and not for our sins only, but also for those of the whole world."
How does St. Thomas measure up to this? Just fine. He teaches us that the purpose of the Incarnation was to take away both original sin and actual sin. But this is not a particularly controversial chapter, except perhaps for certain Calvinists who might deny that Christ was a propitiator not just for Christians but for the whole world. But they must wrestle with the explicit teaching of 1 John 2:2, which Trent has merely quoted:
And he is the propitiation for our sins: and not for ours only, but also for those of the whole world. [emphasis added]