In Lecture 31, Walther discusses mortal vs. venial sins:
In the fifteenth place, the Word of God is not rightly divided when the preacher speaks of certain sins as if they were not of a damnable, but of venial nature.
Unless you ponder the highly important matter now before us well, you will lack much of the clear vision that you ought to have for the proper discharge of the ministerial office.
We have already seen that a distinction must be made between mortal and venial sins. A person failing to make this distinction does not rightly divide Law and Gospel. But the distinction between these two kinds of sin must be made with great care. It must be clearly shown that the distinction is made for the purpose of proving that certain sins expel the Holy Ghost from the believer. When the Holy Spirit is driven out, faith, too, is ejected; for no one can come to faith nor retain it without the Holy Ghost. Sins which expel the Holy Ghost and bring on spiritual death are called mortal sins. Any one who has been a Christian will readily perceive when the Holy Spirit has departed from him by his inability to offer up childlike prayers to God and to resist sin stoutly and bravely as he used to do. He will feel as if he had become chained to sin, like a slave. It is a good thing if he has at least this knowledge of his condition, for thus he may be brought back to God. But while this condition endures, he is not in communion with God.
Venial sins are termed such as a Christian commits without forfeiting the indwelling of the Holy Spirit. They are sins of weakness or rashness; frequently they are called the daily sins of Christians.
While inculcating this distinction upon our hearers, we must be scrupulously careful not to create the notion in them that venial sins are sins about which a person need not be greatly concerned and for which he does not have to ask forgiveness. A preacher who leads his hearers to entertain this view becomes the cause of their perdition. He makes them carnally secure and drives the fear of God from their hearts. That is not the true evangelical way of preaching about these sins, nor is it, in general, a true evangelical notion that only he is a real evangelical preacher who does not preach the Law a great deal. Both the Law and the Gospel must be preached, the one in its sternness, the other in its sweetness. A preacher who does not preach both does not deserve the name of an evangelical minister, but is a false leader and is sowing the Gospel as if he were casting wheat into the ocean, where no crop can be raised. It happens only too often that preachers, when speaking of the distinction between venial and mortal sins, create the impression that to Christians venial sins are matters over which they need not worry. Since all are sinners and no one ever gets rid of sin entirely, there is no reason why one should feel disturbed because of these sins. A talk of that kind is really awful and ungodly.
Christian experience also proves that in its nature no sin is venial. Any true Christian will tell you this to be his experience, that, as soon as he had sinned, he felt an unrest, which continued until he had asked God for forgiveness. In every true Christian the conscience promptly rings an alarm. A Christian merchant becomes restless over five cents in his receipts that do not belong to him. A Christian is reproved by his conscience for wrongdoing when he has treated a brother discourteously or in loveless fashion. For the slightest offense which he has given by his sinful conduct he apologizes, and he has no rest until he has done so. Is not that remarkable? It shows that venial sins, too, are something evil, a fire that may be kindled for our perdition. Small sins become great when they are regarded as small.