Man is justified by the virtues, since "justice is all virtue," as the Philosopher states (Ethic. v, 1). Now man is justified by faith according to Romans 5:1: "Being justified therefore by faith let us have peace," etc. Therefore faith is a virtue. [ST II-II, Q4, A5]
It needs to be said, however, that the faith Aquinas has in view here is not the fiduciary thing held by Protestants.
As shown above, it is by human virtue that human acts are rendered good; hence, any habit that is always the principle of a good act, may be called a human virtue. Such a habit is living faith. For since to believe is an act of the intellect assenting to the truth at the command of the will, two things are required that this act may be perfect: one of which is that the intellect should infallibly tend to its object, which is the true; while the other is that the will should be infallibly directed to the last end, on account of which it assents to the true: and both of these are to be found in the act of living faith. For it belongs to the very essence of faith that the intellect should ever tend to the true, since nothing false can be the object of faith, as proved above (Question 1, Article 3): while the effect of charity, which is the form of faith, is that the soul ever has its will directed to a good end. Therefore living faith is a virtue. [ibid.; emphasis added]
Faith isn't primarily trust; it is better characterized as assent.
Now because this assent requires an act of the will, and because man has a free will, it must be said that this is something that we must do: we must have faith. We must believe the truth.
If the Protestant is going to say that this makes the Gospel "works-based," then it seems to me that he is in the same boat. He says that he must have faith as well (the fact that the faith in view is somewhat different in kind is not really relevant); consequently if a Catholic's faith is "works-based" then so too is the Protestant's.