It is unquestionably the case that nothing I or anyone says will satisfy every critic. So I don't expect to be able to sway every contrary opinion. Nevertheless I think that this charge is likewise false in the present case, as the following (I think) sufficiently demonstrates.
It is written (Romans 8:33): "God that justifieth." Since, then, the inward effect of all the sacraments is justification, it seems that God alone works the interior sacramental effect. [ST III, Q64, A1]
If God alone works the sacramental effect, then nothing accomplished by either the minister or recipient of Baptism plays any part in it. Consequently it is false to say on this account that the Gospel is works-based.
There are two ways of producing an effect; first, as a principal agent; secondly, as an instrument. In the former way the interior sacramental effect is the work of God alone: first, because God alone can enter the soul wherein the sacramental effect takes place; and no agent can operate immediately where it is not: secondly, because grace which is an interior sacramental effect is from God alone, as we have established in I-II, 112, 1; while the character which is the interior effect of certain sacraments, is an instrumental power which flows from the principal agent, which is God. [ibid.; emphasis added]
St. Thomas here explains what he had previously said. Only God can enter the soul so as to produce justification, and grace comes from God alone. Hence it is not possible for any other thing to produce the effect of justification in us. It's also worth nothing in this context that it seems doubtless that Aquinas has a view of justification in mind that is entirely unlike the "imputation" model of Protestantism: if justification is no more than a merely judicial declaration that a man is "just," there would be no need for God to enter the soul to accomplish this. There is no actual internal effect at all in this case. In contrast St. Thomas says that there is, which implies the infusion of righteousness.
In the second way, however, the interior sacramental effect can be the work of man, in so far as he works as a minister. For a minister is of the nature of an instrument, since the action of both is applied to something extrinsic, while the interior effect is produced through the power of the principal agent, which is God. [ibid.]
In the second way the obvious analogy is to some hand tool or other: it accomplishes nothing on its own; it's only effective in the hand of the one who wields it to accomplish some task. It would be ludicrous for us to credit the paintbrush with the beauty of the painting, for example. In the same way, it would be silly to say that the minister of Baptism is the one who justifies a man, since he can no more enter the soul than a brush can apply paint to canvas on its own. Isaiah 10:15.