Lutheran theologians have used those terms. Our class notes at Wisconsin Lutheran Seminary acknowledge that the terms “mortal” and “venial” can be useful. They are useful in identifying two truths: 1) Willful sinning destroys faith; and 2) Christians do not fall from faith with every sin.

It’s important to note that Lutherans don’t use the terms mortal and venial to categorize specific sinful actions as serious or not-so-serious. Instead we view sin from the perspective of the spiritual state of the one who has sinned. Since every sin shatters God’s law (James 2:10), every sin is mortal for an unbeliever. Paul plainly warns: “If you live according to the sinful nature, you will die” (Romans 8:13).

But a believer clings to forgiveness in Christ. By the word “venial,” we speak of sins of weakness or ignorance in the heart and life of a believer. Even though all those sins are damnable by merit, those who take refuge in Christ know they have pardon and forgiveness by Christ’s merits.

As you indicated, the problem with the terms comes from their abuse in Roman Catholicism. Here are definitions from the Catechism of the Catholic Church:

Mortal sin: A grave infraction of the law of God that destroys the divine life in the soul of the sinner (sanctifying grace), constituting a turn away from God. For a sin to be mortal, three conditions must be present: grave matter, full knowledge of the evil of the act, and full consent of the will (p. 889).

Venial sin: Sin which does not destroy the divine life in the soul, as does mortal sin, though it diminishes and wounds it. Venial sin is the failure to observe necessary moderation, in lesser matters of moral law, or in grave matters acting without full knowledge or complete consent (p. 902).

We agree about the danger of sinning with “full knowledge” and “full consent.” But unbiblical confusion is caused when specific sinful actions are categorized as “grave” or “lesser matters.” Such categorizing lulls the sinner to sleep about the seriousness of every sin.

If we believe that small sins don’t matter, we minimize sin and its consequences. As C. F. W. Walther commented in The Proper Distinction Between Law and Gospel: “Small sins become great when they are regarded as small” (p. 332). If I treat any sin of heart or life as venial and therefore harmless, I soon may believe I don’t need Christ and his forgiveness.