Objection 1. It would seem that venial sin is not expiated by the pains of Purgatory as regards the guilt. For a gloss [St. Gregory, Moral. xvi, 28] on 1 John 5:16, "There is a sin unto death," etc. says: "It is vain to ask pardon after death for what was not amended in this life." Therefore no sin is remitted as to guilt after this life.
Objection 2. Further, the same subject is freed from sin as falls into sin. But after death the soul cannot sin venially. Therefore neither can it be loosed from venial sin.
Objection 3. Further, Gregory says [Dial. iv, 39] that every man will be at the judgment as he was when he left the body, because "the tree... wheresoever it shall fall, there shall it be" [Ecclesiastes 11:3]. If, then, a man go forth from this life with venial sin, he will be with venial sin at the judgment: and consequently one does not atone for venial sin in Purgatory.
Objection 4. Further, it has been stated (Supplement, 2, 3) that actual sin is not blotted out save by contrition. But there will be no contrition after this life, because it is a meritorious act. For then there will be neither merit nor demerit since, according to the Damascene [De Fide Orth. ii, 4], "death is to men what the fall was to the angels." Therefore, after this life, venial sin is not remitted in Purgatory as to its guilt.
Objection 5. Further, venial sin is not in us except on account of the fomes. Wherefore in the original state Adam would not have sinned venially, as was stated (Sent. ii, D, xxi, 2). Now after this life there will be no sensuality; because the fomes will cease when the soul is separated, since it is called the "law of the flesh" (Romans 7). Hence there will be no venial sin then, and consequently it cannot be expiated by the fire of Purgatory.
On the contrary, Gregory [Dial. iv, 39] and Augustine [De vera et falsa poenit. iv, xviii, by some other author] say that certain slight sins will be remitted in the life to come. Nor can this be understood of the punishment: because thus all sins, however grave they be, are expiated by the fire of Purgatory, as regards the debt of punishment. Therefore venial sins are cleansed by the fire of Purgatory as to their guilt.
Further, wood, hay, stubble (1 Corinthians 3:12) denote venial sins, as we have said (I-II, 89, 2). Now wood, hay, stubble are consumed in Purgatory. Therefore venial sins are remitted after this life.
I answer that, Some have asserted that no sin is remitted after this life, as regards the guilt: that if a man die with mortal sin, he is damned and incapable of being forgiven; and that it is not possible for a man to die with a venial sin and without mortal sin, since the final grace washes the venial sin away. They assign as reason for this that venial sin is excessive love of a temporal thing, in one who has his foundation in Christ, which excess results from the corruption of concupiscence. Wherefore if grace entirely overcome the corruption of concupiscence, as in the Blessed Virgin, there is no room for venial sin. Hence, since this concupiscence is altogether abated and removed, the powers of the soul are wholly subject to grace, and venial sin is cast out. But this opinion is nonsensical in itself and in its proof. In itself, because it is opposed to the statements of holy men and of the Gospel, which cannot be expounded as referring to the remission of venial sins as to their punishment, as the Master says in the text [Sentent. iv, D, xxi] because in this way both light and grave sins are remitted in the life to come: while Gregory [Dial. iv, 39] declares that light sins alone are remitted after this life. Nor does it suffice for them to say, that this is said expressly of light sins, lest we should think that we shall suffer nothing grievous on their account: because the remission of sin diminishes punishment rather than aggravates it. As to the proof, it is shown to be worthless, since bodily defect, such as obtains at the last moment of life, does not remove the corruption of concupiscence; nor does it diminish it in its root but in its act, as instanced in those who lie dangerously ill; nor again does it calm the powers of the soul, so as to subject them to grace, because tranquillity of the powers, and their subjection to grace, is effected when the lower powers obey the higher which delight together in God's law. But this cannot happen in that state, since the acts of both kinds of powers are impeded; unless tranquillity denote the absence of combat, as occurs even in those who are asleep; and yet sleep is not said, for this reason, to diminish concupiscence, or to calm the powers of the soul, or to subject them to grace. Moreover, granted that the aforesaid defect diminish concupiscence radically, and that it subject the powers to grace, it would still be insufficient to wash away venial sin already committed, although it would suffice in order to avoid it in the future. Because actual sin, even if it be venial, is not remitted without an actual movement of contrition, as stated above (Supplement, 2, 3), however much the latter be in the habitual intention. Now it happens sometimes that a man dies in his sleep, being in a state of grace and yet having a venial sin when he went to sleep: and such a man cannot make an act of contrition for his venial sin before he dies. Nor may we say, as they do, that if he repented neither by act nor by intention, neither in general nor in particular, his venial sin becomes mortal, for that "venial becomes mortal when it is an object of complacency"; because not all complacency in venial sin makes it mortal (else all venial sin would be mortal, since every venial sin pleases for as much as it is voluntary), but only that complacency which amounts to enjoyment, wherein all human wickedness consists, in that "we enjoy what we should use," as Augustine says [De Trin. x, 10]. Hence the complacency which makes a sin mortal is actual complacency, for every mortal sin consists in an act. Now it may happen that a man, after committing a venial sin, has no actual thought of being forgiven or of remaining in that sin, but thinks perhaps about a triangle having its three angles equal to two right angles, and while engaged in this thought falls asleep, and dies.
It is therefore clear that this opinion is utterly unreasonable: and consequently we must say with others that venial sin in one who dies in a state of grace, is remitted after this life by the fire of Purgatory: because this punishment so far as it is voluntary, will have the power, by virtue of grace, to expiate all such guilt as is compatible with grace.
Reply to Objection 1. The gloss refers to mortal sin. Or it may be replied that although, in this life, it is not amended in itself, it is amended in merits, because a man merited here that his punishment should be meritorious to him there.
Reply to Objection 2. Venial sin arises from the corruption of the fomes, which will no longer be in the separate soul that is in Purgatory, wherefore this soul cannot sin venially. On the other hand, the remission of venial sin proceeds from the will informed by grace, which will be in the separate soul in Purgatory. Hence the comparison fails.
Reply to Objection 3. Venial sins do not alter a man's state, for they neither destroy nor diminish charity, according to which the amount of the soul's gratuitous goodness is measured. Hence the soul remains such as it was before, notwithstanding the remission or commission of venial sins.
Reply to Objection 4. After this life there can be no merit in respect of the essential reward, but there can be in respect of some accidental reward, so long as man remains in the state of the way, in a sense. Consequently in Purgatory there can be a meritorious act in respect of the remission of venial sin.
Reply to Objection 5. Although venial sin arises from the proneness of the fomes, sin results in the mind; wherefore even when the fomes is no more, sin can still remain.
Source: Summa Theologica (New Advent)