St. Thomas Aquinas
“Two things may be considered in this sacrament, to wit, the sacrament itself, and the reality of the sacrament: and it appears from both that this sacrament has the power of forgiving venial sins. For this sacrament is received under the form of nourishing food. Now nourishment from food is requisite for the body to make good the daily waste caused by the action of natural heat. But something is also lost daily of our spirituality from the heat of concupiscence through venial sins, which lessen the fervor of charity, as was shown in the SS, Q[24], A[10]. And therefore it belongs to this sacrament to forgive venial sins. Hence Ambrose says (De Sacram. v) that this daily bread is taken ‘as a remedy against daily infirmity.'”
Source: St. Thomas Aquinas, Summa Theologica