An excerpt from Walther…
Preparing to write a sermon which he is to deliver from his pulpit, a minister should approach this task every time with fear and trembling, that is, with the reverent concern that he preach nothing contrary to the Word of God. He must most carefully examine everything that he has written down to see whether it is in harmony with the Word of God and the experience of Christians. He should weigh everything that he is to speak in public in the scales of the sanctuary for weighing gold to see whether it agrees with the writings of the apostles and prophets.
A preacher, after writing a few paragraphs, may be impressed with the beauty and power of what he has written and think that he has succeeded well in his effort. Yet he must not allow that impression to delude him, but he ought carefully once more to go over the very passages which seem so beautiful to him to see whether they contain anything that is false or that has been expressed in such a manner as to be liable to be misunderstood and to arouse false conceptions in his hearers. As soon as he notices something of this kind, he must be stern, yea, cruel, against himself and draw a heavy black line through the beautiful periods, even if he has bestowed much time and labor upon them. Those periods represent labor lost because they were merely the product of his genius, not of a clear knowledge drawn from the Word of God. Indeed, a preacher may discover with considerable alarm that an entire part of his sermon, or even the entire sermon, has turned out altogether wrong.
In a case like that he must not say that he cannot afford to have spent so much labor in vain. If the product turned out wrong, it must be cashiered. There are no two ways about this. If he has no time to write a new sermon, he had better speak rather extempore than deliver what he has laboriously composed. If a minister who is otherwise conscientious has had the misfortune of putting something into his manuscript that is wrong and even saying it from the pulpit, he must, if he notices his mistake while preaching, immediately correct himself and tell his hearers that he really did not mean to say what they have just heard from him. If he notices his mistake later and the matter is of considerable importance, he must make the correction later, lest his hearers be led utterly astray. Yea, he may not only have to correct his wrong statement, but solemnly to revoke it. That will not lower him in the esteem of his listeners; on the contrary, his conscientious striving for accuracy will rather impress them favorably. He must not rely on the ability of his hearers to give the correct interpretation to incorrect statements of his, but must speak so as not to be misunderstood in what he says.
For this reason the apostle addresses this warning to all preachers: "Know ye not that a little leaven leaveneth the whole lump?" 1 Cor. 5, 6; comp. Gal. 5, 9. False teaching is a leaven, yea, we might say, a poison, that will penetrate every artery and kill a person. It is a fact experienced every day that a little poison produces awful effects; it may prove fatal to a person; while a whole piece of arsenic may be swallowed without injury because it becomes enwrapped in saliva. Unspeakably great damage may be caused by one false sentence spoken in a sermon. For instance, the preacher may administer an unwarranted reproof that is taken up by godly, conscientious Christians who are full of concern about their soul and are working out their salvation with fear and trembling. When such sincere Christians observe in themselves something that the preacher has marked as a fault and as something by which men may forfeit divine grace, they may become uncertain of their state of grace and imagine that they dare not believe that they will be saved.
In a case like this the preacher must not think that by talking on the same topic in a different strain on some future Sunday he will have furnished the needed corrective. For the greater the confidence which his hearers have in his orthodoxy, his genuine Christianity, and his great experience, the greater will be their difficulty in plucking out of their hearts the arrow which he shot into them by his unwarranted rebuke. Again, on an occasion when he should have administered a rebuke, he may have offered false comfort to the delight of all false Christians, who disregard all the rest of his sermon and lay hold of only that part which permits them to regard themselves as good Christians, when they are anything but that. Is it not an awful condition for a carnally secure person to get into and to remain in his blindness until the final summons consigns him to eternal perdition?
Mistakes like these may happen even to a sincere pastor. In a moment of inattention, when he is not on his guard and does not pray while he is writing his sermon, God may permit him to rely on his own strength in order to make him see the sorry results which he has achieved without prayer. Imagine the anguish of a minister who has to blame himself when he sees some parishioner of his walking in a wrong path! Every one of your sermons must be the product of heartfelt prayer. When you sit down to the task of writing your sermon and feel that you are distracted, cold, and dead, you must not think: "That cannot be helped; I must fill this page." No; lay your pen aside; call earnestly upon your Father in heaven to lift you out of your miserable state of mind, to give you a fervent heart, to overcome everything in you that is not godly, to let the breath of His Holy Spirit enter your heart, and you will be able to do more than merely write down words of comfort whose import you do not at all feel and which leave your own heart cheerless. You will not indulge in the futile thought that all is well with regard to your sermon since you are only repeating what is in the Bible. Your most serious purpose while preparing your sermon will be to find a way of making a goodly haul with the Gospel net.
- excerpt from C.F.W. Walther, 'Law and Gospel: Twenty-eighth Evening Lecture' (May 15th, 1885)