St. Thomas on Justification - Expiation of sins is through Christ alone

In discussing the fact that the ceremonial laws of the Old Testament had no power to justify anyone, St. Thomas says the following:

"If there had been a law given which could justify, Christ died in vain," i.e. without cause. But this is inadmissible. Therefore the ceremonies of the Old Law did not confer justice.

I answer that, As stated above (102, 5, ad 4), a twofold uncleanness was distinguished in the Old Law. One was spiritual and is the uncleanness of sin. The other was corporal, which rendered a man unfit for divine worship; thus a leper, or anyone that touched carrion, was said to be unclean: and thus uncleanness was nothing but a kind of irregularity. From this uncleanness, then, [namely, the corporal kind - RdP] the ceremonies of the Old Law had the power to cleanse: because they were ordered by the Law to be employed as remedies for the removal of the aforesaid uncleannesses which were contracted in consequence of the prescription of the Law. Hence the Apostle says (Hebrews 9:13) that "the blood of goats and of oxen, and the ashes of a heifer, being sprinkled, sanctify such as are defiled, to the cleansing of the flesh." And just as this uncleanness which was washed away by such like ceremonies, affected the flesh rather than the soul, so also the ceremonies themselves are called by the Apostle shortly before (Hebrews 9:10) justices of the flesh: "justices of the flesh," says he, "being laid on them until the time of correction."

On the other hand, they had no power of cleansing from uncleanness of the soul, i.e. from the uncleanness of sin. The reason of this was that at no time could there be expiation from sin, except through Christ, "Who taketh away the sins of the world" (John 1:29). [ST I-II, Q103, A2; emphasis added]

There is no means by which we can take away sins ourselves. It can only be done by Christ. But this means that we cannot save ourselves, and that we are saved entirely by Christ. Hence those who say that the Catholic Gospel is works-based are badly misinformed.