Objection 1. It would seem that no movement of the free-will towards sin is required for the justification of the ungodly. For charity alone suffices to take away sin, according to Proverbs 10:12: "Charity covereth all sins." Now the object of charity is not sin. Therefore for this justification of the ungodly no movement of the free-will towards sin is required.
Objection 2. Further, whoever is tending onward, ought not to look back, according to Philippians 3:13-14: "Forgetting the things that are behind, and stretching forth myself to those that are before, I press towards the mark, to the prize of the supernal vocation." But whoever is stretching forth to righteousness has his sins behind him. Hence he ought to forget them, and not stretch forth to them by a movement of his free-will.
Objection 3. Further, in the justification of the ungodly one sin is not remitted without another, for "it is irreverent to expect half a pardon from God" [Cap., Sunt. plures: Dist. iii, De Poenit.]. Hence, in the justification of the ungodly, if man's free-will must move against sin, he ought to think of all his sins. But this is unseemly, both because a great space of time would be required for such thought, and because a man could not obtain the forgiveness of such sins as he had forgotten. Hence for the justification of the ungodly no movement of the free-will is required.
On the contrary, It is written (Psalm 31:5): "I will confess against myself my injustice to the Lord; and Thou hast forgiven the wickedness of my sin."
I answer that, As stated above (Article 1), the justification of the ungodly is a certain movement whereby the human mind is moved by God from the state of sin to the state of justice. Hence it is necessary for the human mind to regard both extremes by an act of free-will, as a body in local movement is related to both terms of the movement. Now it is clear that in local movement the moving body leaves the term "whence" and nears the term "whereto." Hence the human mind whilst it is being justified, must, by a movement of its free-will withdraw from sin and draw near to justice.
Now to withdraw from sin and to draw near to justice, in an act of free-will, means detestation and desire. For Augustine says on the words "the hireling fleeth," etc. (John 10:12): "Our emotions are the movements of our soul; joy is the soul's outpouring; fear is the soul's flight; your soul goes forward when you seek; your soul flees, when you are afraid." Hence in the justification of the ungodly there must be two acts of the free-will--one, whereby it tends to God's justice; the other whereby it hates sin.
Reply to Objection 1. It belongs to the same virtue to seek one contrary and to avoid the other; and hence, as it belongs to charity to love God, so likewise, to detest sin whereby the soul is separated from God.
Reply to Objection 2. A man ought not to return to those things that are behind, by loving them; but, for that matter, he ought to forget them, lest he be drawn to them. Yet he ought to recall them to mind, in order to detest them; for this is to fly from them.
Reply to Objection 3. Previous to justification a man must detest each sin he remembers to have committed, and from this remembrance the soul goes on to have a general movement of detestation with regard to all sins committed, in which are included such sins as have been forgotten. For a man is then in such a frame of mind that he would be sorry even for those he does not remember, if they were present to his memory; and this movement cooperates in his justification.
Source: Summa Theologica (New Advent)