Objection 1. It would seem that all the remnants of sin are removed when a mortal sin is forgiven. For Augustine says in De Poenitentia [De vera et falsa Poenitentia, the authorship of which is unknown]: "Our Lord never healed anyone without delivering him wholly; for He wholly healed the man on the Sabbath, since He delivered his body from all disease, and his soul from all taint." Now the remnants of sin belong to the disease of sin. Therefore it does not seem possible for any remnants of sin to remain when the guilt has been pardoned.
Objection 2. Further, according to Dionysius (Div. Nom. iv), "good is more efficacious than evil, since evil does not act save in virtue of some good." Now, by sinning, man incurs the taint of sin all at once. Much more, therefore, by repenting, is he delivered also from all remnants of sin.
Objection 3. Further, God's work is more efficacious than man's. Now by the exercise of good human works the remnants of contrary sins are removed. Much more, therefore, are they taken away by the remission of guilt, which is a work of God.
On the contrary, We read (Mark 8) that the blind man whom our Lord enlightened, was restored first of all to imperfect sight, wherefore he said (Mark 8:24): "I see men, as it were trees, walking"; and afterwards he was restored perfectly, "so that he saw all things clearly." Now the enlightenment of the blind man signifies the delivery of the sinner. Therefore after the first remission of sin, whereby the sinner is restored to spiritual sight, there still remain in him some remnants of his past sin.
I answer that, Mortal sin, in so far as it turns inordinately to a mutable good, produces in the soul a certain disposition, or even a habit, if the acts be repeated frequently. Now it has been said above (Article 4) that the guilt of mortal sin is pardoned through grace removing the aversion of the mind from God. Nevertheless when that which is on the part of the aversion has been taken away by grace, that which is on the part of the inordinate turning to a mutable good can remain, since this may happen to be without the other, as stated above (Article 4). Consequently, there is no reason why, after the guilt has been forgiven, the dispositions caused by preceding acts should not remain, which are called the remnants of sin. Yet they remain weakened and diminished, so as not to domineer over man, and they are after the manner of dispositions rather than of habits, like the "fomes" which remains after Baptism.
Reply to Objection 1. God heals the whole man perfectly; but sometimes suddenly, as Peter's mother-in-law was restored at once to perfect health, so that "rising she ministered to them" (Luke 4:39), and sometimes by degrees, as we said above (44, 3, ad 2) about the blind man who was restored to sight (Matthew 8). And so too, He sometimes turns the heart of man with such power, that it receives at once perfect spiritual health, not only the guilt being pardoned, but all remnants of sin being removed as was the case with Magdalen (Luke 7); whereas at other times He sometimes first pardons the guilt by operating grace, and afterwards, by co-operating grace, removes the remnants of sin by degrees.
Reply to Objection 2. Sin too, sometimes induces at once a weak disposition, such as is the result of one act, and sometimes a stronger disposition, the result of many acts.
Reply to Objection 3. One human act does not remove all the remnants of sin, because, as stated in the Predicaments (Categor. viii) "a vicious man by doing good works will make but little progress so as to be any better, but if he continue in good practice, he will end in being good as to acquired virtue." But God's grace does this much more effectively, whether by one or by several acts.
Source: Summa Theologica (New Advent)