Rightly distinguishing the Law and the Gospel is the most difficult and the highest art of Christians in general and of theologians in particular. It is taught only by the Holy Spirit in the school of experience.
Possibly some one among you is thinking, "Is this thesis really true? I have now heard five lectures on this subject, and it is perfectly clear to me. If this is the most difficult art, I know it." But, my dear friend, you are greatly mistaken. Consider that the thesis does not mean that the doctrine of the Law and the Gospel is so difficult that it cannot be learned without the aid of the Holy Ghost. It is easy — easy enough for children to learn. Every child can comprehend this doctrine. It is contained in every catechism. It is not strong meat, but milk. It is the frist letters of the alphabet, it belongs to the rudiments of Christianity; for without this doctrine no person can be a Christian. Even a small child soon learns these facts: "The First Part of the Catechism treats of the Ten Commandments, the Second Part of the Creed. We are first told what we are to do; next, that a person need only believe to be saved." In other words, the child observes that the Second Part does not, like the First, make demands. This doctrine of the distinction of Law and Gospel is entirely different from the doctrine of the attributes by which the three persons in the Godhead are distinct from one another; or the doctrine of predestination with its many inscrutable mysteries, or the doctrine of the communication of the divine attributes to the human nature of Christ. These doctrines exceed the grasp of children and cannot be comprehended by them. But the doctrine of the distinction between the Law and the Gospel is different. You know it now. But at the present time we are studying th application and the use of this doctrine. The practical application of this doctrine presents difficulties which no man can surmount by reasonable reflections. The Holy Spirit must teach men this in the school of experience. The difficulties of mastering this art confront the minister, in the first place, in so far as he is a Christian; in the second place, in so far as he is a minister.
We read Ps. 51, 10. 11: Create in me a clean heart, O God, and renew a right spirit within me. Cast me not away from Thy presence and take not Thy Holy Spirit from me.
It is a characteristic of Christians to regard th Scriptures as the true, infallible Word of God. But when they are in need of comfort, they find none; they cry for mercy; they supplicate God on their knees. God made David taste the bitterness of sin. In general, we behold David after his fall more frequently in sadness than in joyful spirits, and we see that one misfortune after the other befalls him. God did not permit these misfortunes to afflict David because He had not forgiven his sin, but in order to keep him from falling into another sin. It was nothing but love and mercy that prompted God to act thus. Naturally, a person still dead in sin things: "Why was David so foolish as to torment his mind with a sin that had been forgiven by God?" A person reasoning thus makes of the Gospel a pillow for his carnal mind to rest on; he continues his sinful life and imagines that he will, after all, land in heaven. His Gospel is a gospel for the flesh.
- excerpt from C.F.W. Walther, 'Law and Gospel, Sixth Evening Lecture' (1884)