Finally, Christian experience also proves that by its very nature no sin is venial. Any true Christian will tell you: “Yes, I have experienced that myself. As soon as I had sinned. I felt uneasy. That continued until I asked God for forgiveness.” The conscience of every true Christian immediately rings an alarm. A Christian merchant has no peace over five cents in his bookkeeping that do not belong to him.
Their consciences scold Christians for wrongdoing when they treat a brother impolitely or in a loveless fashion. [Christians] apologize for the slightest offense that they have given by any sinful behavior. They do not rest until they have done so. Is that not remarkable? It shows that venial sins, too, are something evil, a fire that can flare up and cast us into damnation. Small sins become great when they are regarded as small.
Law & Gospel: How to Read and Apply the Bible by C. F. W. Walther (A Reader’s Edition)