Thomas Aquinas

“ Nevertheless, since in those who have the use of free-will (in whom alone can there be venial sins) , there can be no infusion of grace without an actual movement of the free-will towards God and against sin, consequently whenever grace is infused anew, venial sins are forgiven.
Reply to Objection 1: Even the forgiveness of venial sins is an effect of grace, in virtue of the act which grace produces anew, but not through any habit infused anew into the soul. ”

“ Consequently it is necessary to have a certain virtual displeasure, so that, for instance, a man's affections so tend to God and Divine things, that whatever might happen to him to hamper that tendency would be displeasing to him, and would grieve him, were he to commit it, even though he were not to think of it actually: and this is not sufficient for the remission of mortal sin, except as regards those sins which he fails to remember after a careful examination. ”

“ Consequently penance for mortal sins requires man to purpose abstaining from mortal sins, all and each; whereas penance for venial sins requires man to purpose abstaining from each, but not from all, because the weakness of this life does not allow of this. Nevertheless he needs to have the purpose of taking steps to commit fewer venial sins, else he would be in danger of falling back, if he gave up the desire of going forward, or of removing the obstacles to spiritual progress, such as venial sins are. ”

“ Therefore forgiveness of venial sins is caused by the fervor of charity, which may be without actual displeasure at venial sin.
On the contrary, Augustine says in De Poenitentia [*De vera et falsa Poenitentia, the authorship of which is unknown] , that "there is a penance which is done for venial sins in the Church every day" which would be useless if venial sins could be forgiven without Penance.
I answer that, Forgiveness of sin, as stated above ( [4777] Q [86] , A [2] ) , is effected by man being united to God from Whom sin separates him in some way. ”

“ 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. ”