Question: What is the difference between mortal and venial sin?
Answer: Sin is a deliberate thought, word, action, or omission contrary to God’s law. Since the beginnings of the Church, sin has been distinguished by its gravity. “Grave matter” is traditionally defined by the Ten Commandments. If a person commits a grave sin with full knowledge of the action’s evil and with the complete consent of their will, this is a mortal sin. The effects of mortal sin are grave — the loss of sanctifying grace, which can be restored by a fresh outpouring of God’s mercy in the sacrament of Reconciliation.
Venial sin does not fit one or more of these conditions (for example, a less serious matter still freely chosen or grave matter committed while under force or ignorance of God’s law). The effects of venial sin — while still damaging to the human heart and human society — are less severe. Even so, smaller sins build habits, which can obstruct our growth in virtue. When we are serious about following Jesus, we should do our best to avoid all sin, whether large or small!