"The most necessary condition for the remission of any sin, and therefore also of venial sin, is contrition. So long as a man is attached to sin and does not detest it, God cannot forgive it, for He is infinitely holy and just. . .
All the Sacraments; they remit sins ex opere operato, and especially those sins which are opposed to the particular end of the Sacrament. For the object of every Sacrament is the sanctification of souls, and hence the removal of all that hinders this sanctification. Not all the Sacraments, however, effect this forgiveness in the same manner. . .
[Directly, indirectly, etc.]
It is always, however, necessary and sufficient to elicit at least a virtual or implicit contrition, contained in a pious and supernatural affection toward God, which is opposed to venial sins, and is consequently a virtual horror and retraction of the same.
The sacramentals also destroy venial sins. “By the use of the sacramentals the faithful confess and awaken their faith, hope, reverence for God, a longing for interior holiness and sinlessness, or a horror of sin, and sorrow for past offences. The symbols blessed or used by the Church confer a grace which produces or strengthens desires and acts of different virtues, which in turn destroy venial sin and atone for it.” Hence a sacramental possesses power of remitting sin in proportion as its character and the blessing of the Church cause it to excite or strengthen acts of virtue in the faithful. The Church has a sacramental especially designed for the remission of venial sins, and makes use of it on those occasions when the faithful need greatest purity of heart. It consists of the two prayers: Misereatur vestri, etc., and Indulgentiam, absolutionem, etc. To these we may add the use of holy water, which, in accordance with the intention and prayers of the Church when she blesses it, is designed to ward off the devil s influence from animate and inanimate creatures and to protect them from impurity, sickness, and harm. The effect of the other sacramentals in procuring remission of venial sins is not so direct. The more they are of their own nature suited to awaken contrition, and the more directly the intention in the use of them is directed to the cleansing from sin, so much the more effectual are they in this respect. "
- Theory & Practice of the Confessional