We have already seen, with regards to ST I-II, Q103, A2, that Aquinas taught us the fact that sins are only expiated through Christ. The reason of this necessity, he tells us in the same place, is that there is no law given by which our sins may be justified before God.
"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. [emphasis added]There is no means by which something we do ourselves can justify us before God; he has given us no such law. Part of the reason for this is that it is beyond our ability to attain to our last end by anything we do ourselves, as we have also seen. We cannot keep the moral law by our own strength, as we have seen, and we cannot be justified by anything we do either; so we see that our salvation is entirely from God.