Why Do Catholics Believe In Venial and Mortal Sins?

The gates of hell are open, night and day; smooth the descent, and easy the way.
Virgil, The Aeneid

The Catholic Church divides sins into two groups: venial sins and mortal sins. St. John makes a clear distinction between two essentially different types of sin: a sin that leads to death, and a sin that does not lead to death.¹ I used to believe the level of wickedness applied to a variety of sins is fixed to the same degree; essentially every sin is the same in God’s eyes. Predominantly, Protestant theology makes no distinction between mortal and venial sins; their propensity is to regard all sins as either mortal sin, each sin deserving eternal damnation, or all sins as a venial sin, heaven bound despite repetitious sinning.² If every sin were a mortal sin, we would lose our salvation every day; and if every sin were a venial sin our relationship with God would be autonomous of what we say and do in relation to others and to God—both views treat salvation as fundamentally impersonal. The Catechism of the Catholic Church states, “Mortal sin destroys charity in the heart of man by a grave violation of God’s law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him. Venial sin allows charity to subsist, even though it offends and wounds it.”³

Sin is an offense contrary to reason—an act diametrically opposed to our salvation resulting in a revolt against God. Disregarding the differences between mortal and venial sins have had damaging consequences to the Christian faith—I’m not entirely pointing at my Protestant brothers and sisters—Christians, in general, have disregarded this truth, allowing formidable errors in ethical perception. The weight of man’s free will allows the possibility of sin, as much as the possibility of love, thus we are unequivocally accountable for either choice of behavior. In Mere Christianity C.S. Lewis eloquently describes the principles of the moral law: “If we do not believe in decent behaviour, why should we be so anxious to make excuses for not having behaved decently? The truth is, we believe in decency so much—we feel the Rule of Law pressing on us so— that we cannot bear to face the fact that we are breaking it, and consequently we try to shift the responsibility. For you notice that it is only for our bad behaviour that we find all these explanations.”

“Three things are necessary for the salvation of man: to know what he ought to believe; to know what he ought to desire; and to know what he ought to do.” — St. Thomas Aquinas

What has always mattered, is whether or not there is agape in the heart; only the heart having living faith will have the “righteousness of faith”. By committing a mortal sin, with a comprehensive understanding, a person chooses to act not out of love for God but for themselves—thus removing a person from a state of grace. When we order our lives to God as our highest end (e.g. loving God with all our heart), we receive grace and agape. Our justification is by living faith, not by dead works so that we might walk in newness of life. It would be anthropomorphic to assume that God wouldn’t allow us to turn away from Him—love and sin are evidence of free will.

¹Peter 4:8; 1 John 5:16-17
²“There is no sin so small, but it deserves damnation.” John Calvin, WCF XV.4
³CCC 1855