Objection 1. It would seem that venial sin causes a stain in the soul. For Augustine says (De Poenit.) [Hom. 50, inter. L., 2, that if venial sins be multiplied, they destroy the beauty of our souls so as to deprive us of the embraces of our heavenly spouse. But the stain of sin is nothing else but the loss of the soul's beauty. Therefore venial sins cause a stain in the soul.
Objection 2. Further, mortal sin causes a stain in the soul, on account of the inordinateness of the act and of the sinner's affections. But, in venial sin, there is an inordinateness of the act and of the affections. Therefore venial sin causes a stain in the soul.
Objection 3. Further, the stain on the soul is caused by contact with a temporal thing, through love thereof as stated above (Question 86, Article 1). But, in venial sin, the soul is in contact with a temporal thing through inordinate love. therefore, venial sin brings a stain on the soul.
On the contrary, it is written, (Ephesians 5:27): "That He might present it to Himself a glorious church, not having spot or wrinkle," on which the gloss says: "i.e., some grievous sin." Therefore it seems proper to mortal sin to cause a stain on the soul.
I answer that as stated above (Question 86, Article 1), a stain denotes a loss of comeliness due to contact with something, as may be seen in corporeal matters, from which the term has been transferred to the soul, by way of similitude. Now, just as in the body there is a twofold comeliness, one resulting from the inward disposition of the members and colors, the other resulting from outward refulgence supervening, so too, in the soul, there is a twofold comeliness, one habitual and, so to speak, intrinsic, the other actual like an outward flash of light. Now venial sin is a hindrance to actual comeliness, but not to habitual comeliness, because it neither destroys nor diminishes the habit of charity and of the other virtues, as we shall show further on (II-II, 24, 10; 133, 1, ad 2), but only hinders their acts. On the other hand a stain denotes something permanent in the thing stained, wherefore it seems in the nature of a loss of habitual rather than of actual comeliness. Therefore, properly speaking, venial sin does not cause a stain in the soul. If, however, we find it stated anywhere that it does induce a stain, this is in a restricted sense, in so far as it hinders the comeliness that results from acts of virtue.
Reply to Objection 1. Augustine is speaking of the case in which many venial sins lead to mortal sin dispositively: because otherwise they would not sever the soul from its heavenly spouse.
Reply to Objection 2. In mortal sin the inordinateness of the act destroys the habit of virtue, but not in venial sin.
Reply to Objection 3. In mortal sin the soul comes into contact with a temporal thing as its end, so that the shedding of the light of grace, which accrues to those who, by charity, cleave to God as their last end, is entirely cut off.
On the contrary, in venial sin, man does not cleave to a creature as his last end: hence there is no comparison.
Source: Summa Theologica (New Advent)