It is written (Galatians 4:9): "Turn you again to the weak and needy elements?" i.e. "to the Law," says the gloss, "which is called weak, because it does not justify perfectly." But grace justifies perfectly. Therefore the sacraments of the old Law did not confer grace. [ST III, Q62, A6; emphasis added]
For purposes of the present series on justification, the relevant portion is that grace justifies perfectly. Now if grace justifies perfectly, then there is no aspect of our justification which must be perfected by anything that we might do. Consequently we see once again that we are saved not by works (as some falsely allege) but rather by grace.
But back to the article in question. Responding to an objection that at least circumcision was a means of grace, St. Thomas replies:
There have been many opinions about Circumcision. For, according to some, Circumcision conferred no grace, but only remitted sin. But this is impossible; because man is not justified from sin save by grace, according to Romans 3:24: "Being justified freely by His grace."
Wherefore others said that by Circumcision grace is conferred, as to the privative effects of sin, but not as to its positive effects. But this also appears to be false, because by Circumcision, children received the faculty of obtaining glory, which is the ultimate positive effect of grace. Moreover, as regards the order of the formal cause, positive effects are naturally prior to privative effects, though according to the order of the material cause, the reverse is the case: for a form does not exclude privation save by informing the subject.
Hence others say that Circumcision conferred grace also as regards a certain positive effect, i.e. by making man worthy of eternal life, but not so as to repress concupiscence which makes man prone to sin. And so at one time it seemed to me. But if the matter be considered carefully, this too appears to be untrue; because the very least grace is sufficient to resist any degree of concupiscence, and to merit eternal life.
And therefore it seems better to say that Circumcision was a sign of justifying faith: wherefore the Apostle says (Romans 4:11) that Abraham "received the sign of Circumcision, a seal of the justice of faith." Consequently grace was conferred in Circumcision in so far as it was a sign of Christ's future Passion, as will be made clear further on (70, 4). [ibid., ad 3; emphasis added]
Here we see (in the highlighted portion) that Aquinas emphatically denies any possibility that we are justified by anything other than grace. This being the case, it's clear that it's flatly untrue to claim (as some do) that Catholics believe in a "works-based" gospel. We don't. We are saved by grace alone.