The Redemptorist Father Peter Geiermann explains how penance purifies us of the effects of sin: “As we offend God, defile our souls, and incur the penalty of sin by rebelling against God, so we honor God, purify our consciences, and make satisfaction for sin by voluntary penance.” He lists six causes for penance, that is, six things which occasion the practice of penance. These are: mortal sin, venial sin, tepidity, participation in the sins of others, the sins of the world, and personal imperfections.
“Mortal sin is a deliberate, voluntary transgression of God’s law in a serious matter. Its malice consists in rebelling against God with base contempt and vile ingratitude. . . . Its effects are: (1) it insults the majesty and goodness of God; (2) it deprives man of sanctifying grace, and past merit, and renders him incapable to merit heaven; (3) it defiles the soul; (4) it burdens the conscience with guilt; (5) it condemns the sinner to the punishment of an outraged conscience, the loss of God’s friendship, and brings upon him the punishment of hell.”
“A venial sin is (1) a deliberate, voluntary transgression of God’s law in a light matter, or (2) a transgression in a serious matter where the mind was prevented from realizing the extent of its malice through ignorance, misconception or inadvertence, or where mind or will were momentarily hampered, but not paralyzed by sudden passion, nervousness, or fear. Where physical violence, however, or nervousness, fear, or antecedent passion deprive man entirely of the dominion of an action, it is no sin in the sight of God. The effects of venial sin are: (1) it slights God; (2) it lessens the fear of the Lord and the love of God in our hearts; (3) it renders us unworthy of special graces; (4) it diminishes our fervor and devotion; (5) it disposes us for the commission of mortal sin; (6) it makes us deserving of temporal punishment.”
Quotations from Peter Geiermann, The Narrow Way (New York: Benziger, 1914).