Luther states that “Law and Gospel must be proclaimed, and the two must not be mingled.” A pastor who is not focused in his preaching preaches himself rather than Christ. But anyone preaching himself preaches people into hell, even when they say of his preaching: “Ah, that was beautiful! That man is an orator!” Even a true, righteous preacher is tempted by vain thoughts that spring from his sinful flesh. But as soon as he notices this, he will cast these cursed thoughts of vanity aside and will carry to God to rid him of them. He enters his pulpit a humble man. People can tell whether his preaching comes from the heart or not.
Of course, you cannot preach like a Luther. Yet you still need to consider: “How can I preach the Law to the secure and, at the same time, the Gospel to crushed sinners?” Every sermon must contain both doctrines. As soon as one of them is missing, the other is wrong. For any sermon is wrong that does not present all that is necessary for a person’s salvation. Do not think that you have done rightly if you generically preach Law in one part of your sermon and Gospel in the other. No. A topical division of this kind is worthless. Both doctrines may even be contained in one sentence. But everyone in your audience must have the impression: “He is preaching to me!” Even the most comforting and cheerful sermon must contain Law as well.
Law & Gospel: How to Read and Apply the Bible by C. F. W. Walther (A Reader’s Edition) p 29