Justification taken passively implies a movement towards justice, as heating implies a movement towards heat. But since justice, by its nature, implies a certain rectitude of order, it may be taken in two ways: first, inasmuch as it implies a right order in man's act, and thus justice is placed amongst the virtues--either as particular justice, which directs a man's acts by regulating them in relation to his fellowman--or as legal justice, which directs a man's acts by regulating them in their relation to the common good of society, as appears from Ethic. v, 1. [ST I-II, Q113, A1; emphasis added]
"Movement towards justice" doesn't imply the instantaneous thing the Protestant believes, in which he reduces justification to a mere judicial decree. No. It implies the acquisition of the thing, "as heating implies a movement towards heat."
Secondly, justice is so-called inasmuch as it implies a certain rectitude of order in the interior disposition of a man, in so far as what is highest in man is subject to God, and the inferior powers of the soul are subject to the superior, i.e. to the reason; and this disposition the Philosopher calls "justice metaphorically speaking" (Ethic. v, 11). Now this justice may be in man in two ways: first, by simple generation, which is from privation to form; and thus justification may belong even to such as are not in sin, when they receive this justice from God, as Adam is said to have received original justice. Secondly, this justice may be brought about in man by a movement from one contrary to the other, and thus justification implies a transmutation from the state of injustice to the aforesaid state of justice. And it is thus we are now speaking of the justification of the ungodly, according to the Apostle (Romans 4:5): "But to him that worketh not, yet believeth in Him that justifieth the ungodly," etc. And because movement is named after its term "whereto" rather than from its term "whence," the transmutation whereby anyone is changed by the remission of sins from the state of ungodliness to the state of justice, borrows its name from its term "whereto," and is called "justification of the ungodly." [ST, op. cit.]
This is not to say that there is no "instantaneous" aspect to justification at all. God's forgiveness is not a thing given progressively. But St. Thomas tells us that the movement towards justice (which is what justification means) is a movement towards "a certain rectitude of order in the interior disposition of a man, in so far as what is highest in man is subject to God."
It's clear from this explanation that St. Thomas is concerned not merely with a judicial condition, but rather with the question of whether the sinner becomes just.
Every sin, inasmuch as it implies the disorder of a mind not subject to God, may be called injustice, as being contrary to the aforesaid justice. [ibid., ad 1]
A judicial declaration is not the same as a removal of this disorder; it does not make the unjust to be holy, to be just. Obviously this doesn't mean that the declaration is irrelevant: far from it! But it does mean that we can't get by on that alone. God doesn't simply declare our sins to be forgiven; by his grace he moves us towards justice - towards actually being holy.
None of this happens because of human action. The Catholic Church does not teach a works-based salvation. God forgives our sins by his grace; God moves us to become just by his grace, and we cannot become just apart from his grace. Those who say that the Catholic Church teaches otherwise are badly misinformed.