Objection 1. It would seem that the justification of the ungodly is not the remission of sins. For sin is opposed not only to justice, but to all the other virtues, as stated above (Question 71, Article 1). Now justification signifies a certain movement towards justice. Therefore not even remission of sin is justification, since movement is from one contrary to the other.
Objection 2. Further, everything ought to be named from what is predominant in it, according to De Anima ii, text. 49. Now the remission of sins is brought about chiefly by faith, according to Acts 15:9: "Purifying their hearts by faith"; and by charity, according to Proverbs 10:12: "Charity covereth all sins." Therefore the remission of sins ought to be named after faith or charity rather than justice.
Objection 3. Further, the remission of sins seems to be the same as being called, for whoever is called is afar off, and we are afar off from God by sin. But one is called before being justified according to Romans 8:30: "And whom He called, them He also justified." Therefore justification is not the remission of sins.
On the contrary, On Romans 8:30, "Whom He called, them He also justified," the gloss says i.e. "by the remission of sins." Therefore the remission of sins is justification.
I answer that, 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.
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."
Reply to Objection 1. 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, according to 1 John 3:4: "Whosoever committeth sin, committeth also iniquity; and sin is iniquity." And thus the removal of any sin is called the justification of the ungodly.
Reply to Objection 2. Faith and charity imply a special directing of the human mind to God by the intellect and will; whereas justice implies a general rectitude of order. Hence this transmutation is named after justice rather than after charity or faith.
Reply to Objection 3. Being called refers to God's help moving and exciting our mind to give up sin, and this motion of God is not the remission of sins, but its cause.
Source: Summa Theologica (New Advent)