Objection 1. It would seem that for the remission of guilt, which is the justification of the ungodly, no infusion of grace is required. For anyone may be moved from one contrary without being led to the other, if the contraries are not immediate. Now the state of guilt and the state of grace are not immediate contraries; for there is the middle state of innocence wherein a man has neither grace nor guilt. Hence a man may be pardoned his guilt without his being brought to a state of grace.
Objection 2. Further, the remission of guilt consists in the Divine imputation, according to Psalm 31:2: "Blessed is the man to whom the Lord hath not imputed sin." Now the infusion of grace puts something into our soul, as stated above (Question 110, Article 1). Hence the infusion of grace is not required for the remission of guilt.
Objection 3. Further, no one can be subject to two contraries at once. Now some sins are contraries, as wastefulness and miserliness. Hence whoever is subject to the sin of wastefulness is not simultaneously subject to the sin of miserliness, yet it may happen that he has been subject to it hitherto. Hence by sinning with the vice of wastefulness he is freed from the sin of miserliness. And thus a sin is remitted without grace.
On the contrary, It is written (Romans 3:24): "Justified freely by His grace."
I answer that, by sinning a man offends God as stated above (Question 71, Article 5). Now an offense is remitted to anyone, only when the soul of the offender is at peace with the offended. Hence sin is remitted to us, when God is at peace with us, and this peace consists in the love whereby God loves us. Now God's love, considered on the part of the Divine act, is eternal and unchangeable; whereas, as regards the effect it imprints on us, it is sometimes interrupted, inasmuch as we sometimes fall short of it and once more require it. Now the effect of the Divine love in us, which is taken away by sin, is grace, whereby a man is made worthy of eternal life, from which sin shuts him out. Hence we could not conceive the remission of guilt, without the infusion of grace.
Reply to Objection 1. More is required for an offender to pardon an offense, than for one who has committed no offense, not to be hated. For it may happen amongst men that one man neither hates nor loves another. But if the other offends him, then the forgiveness of the offense can only spring from a special goodwill. Now God's goodwill is said to be restored to man by the gift of grace; and hence although a man before sinning may be without grace and without guilt, yet that he is without guilt after sinning can only be because he has grace.
Reply to Objection 2. As God's love consists not merely in the act of the Divine will but also implies a certain effect of grace, as stated above (Question 110, Article 1), so likewise, when God does not impute sin to a man, there is implied a certain effect in him to whom the sin is not imputed; for it proceeds from the Divine love, that sin is not imputed to a man by God.
Reply to Objection 3. As Augustine says (De Nup. et Concup. i, 26), if to leave off sinning was the same as to have no sin, it would be enough if Scripture warned us thus: "'My son, hast thou sinned? do so no more?' Now this is not enough, but it is added: 'But for thy former sins also pray that they may be forgiven thee.'" For the act of sin passes, but the guilt remains, as stated above (Question 87, Article 6). Hence when anyone passes from the sin of one vice to the sin of a contrary vice, he ceases to have the act of the former sin, but he does not cease to have the guilt, hence he may have the guilt of both sins at once. For sins are not contrary to each other on the part of their turning from God, wherein sin has its guilt.
Source: Summa Theologica (New Advent)