Objection 1. It would seem that venial sins are not removed by the sprinkling of holy water, a bishop's blessing, and the like. For venial sins are not forgiven without Penance, as stated above (Article 1). But Penance suffices by itself for the remission of venial sins. Therefore the above have nothing to do with the remission of venial sins.
Objection 2. Further, each of the above bears the same relation to one venial sin as to all. If therefore, by means of one of them, some venial sin is remitted, it follows that in like manner all are remitted, so that by beating his breast once, or by being sprinkled once with holy water, a man would be delivered from all his venial sins, which seems unreasonable.
Objection 3. Further, venial sins occasion a debt of some punishment, albeit temporal; for it is written (1 Corinthians 3:12-15) of him that builds up "wood, hay, stubble" that "he shall be saved, yet so as by fire." Now the above things whereby venial sins are said to be taken away, contain either no punishment at all, or very little. Therefore they do not suffice for the full remission of venial sins.
On the contrary, Augustine says in De Poenitentia [Hom. 30 inter 1; Ep. cclxv] that "for our slight sins we strike our breasts, and say: Forgive us our trespasses," and so it seems that striking one's breast, and the Lord's Prayer cause the remission of venial sins: and the same seems to apply to the other things.
I answer that, As stated above (Article 2), no infusion of fresh grace is required for the forgiveness of a venial sin, but it is enough to have an act proceeding from grace, in detestation of that venial sin, either explicit or at least implicit, as when one is moved fervently to God. Hence, for three reasons, certain things cause the remission of venial sins: first, because they imply the infusion of grace, since the infusion of grace removes venial sins, as stated above (Article 2); and so, by the Eucharist, Extreme Unction, and by all the sacraments of the New Law without exception, wherein grace is conferred, venial sins are remitted. Secondly, because they imply a movement of detestation for sin, and in this way the general confession [i.e. the recital of the Confiteor or of an act of contrition, the beating of one's breast, and the Lord's Prayer conduce to the remission of venial sins, for we ask in the Lord's Prayer: "Forgive us our trespasses." Thirdly, because they include a movement of reverence for God and Divine things; and in this way a bishop's blessing, the sprinkling of holy water, any sacramental anointing, a prayer said in a dedicated church, and anything else of the kind, conduce to the remission of venial sins.
Reply to Objection 1. All these things cause the remission of venial sins, in so far as they incline the soul to the movement of penance, viz., the implicit or explicit detestation of one's sins.
Reply to Objection 2. All these things, so far as they are concerned, conduce to the remission of all venial sins: but the remission may be hindered as regards certain venial sins, to which the mind is still actually attached, even as insincerity sometimes impedes the effect of Baptism.
Reply to Objection 3. By the above things, venial sins are indeed taken away as regards the guilt, both because those things are a kind of satisfaction, and through the virtue of charity whose movement is aroused by such things.
Yet it does not always happen that, by means of each one, the whole guilt of punishment is taken away, because, in that case, whoever was entirely free from mortal sin, would go straight to heaven if sprinkled with holy water: but the debt of punishment is remitted by means of the above, according to the movement of fervor towards God, which fervor is aroused by such things, sometimes more, sometimes less.
Source: Summa Theologica (New Advent)