"It pleased God by the foolishness of preaching to save them that believe" (1 Corinthians 1:21).
Mind you, it it the "foolishness of preaching" and not "preaching foolishly."
(excerpt pg. 57-62)
... The whole policy of the pastor's instructions is contained, in germ, in that saying of Christ: "They that be whole need not a physician; but they that be sick." That men may heartily embrace the gospel, the essential point is to make them know and feel their radical disease. This you will not teach them effectually by mere general announcements of depravity and the fall. When the claims of the law are brought to the souls, when they are made to see perspicuously what is their extent, and that they are reasonable, when they become conscious of their own innate and fundamental enmity to those just demands, and bondage to evil desires; and when they hear the wrath denounced by God against every transgression; then there is hope that they will find themselves truly lost, and will cry to the Deliverer for rescue.
But, second: the practical definition of Christianity has been fully accepted by us. Its end and aim is holy living. Of this holy life, the law of God is the rule. The believer justified in Christ does not, indeed, look to the law for his redeeming merit; but he receives it as his guide to the obedience of faith and love, as fully as though he were still under a covenant of works. He therefore needs practical instruction, as really as the unbeliever. It must stimulate and direct him in the Christian race, and make him a "peculiar person, zealous of good works." The exclusive preaching of doctrine to professed Christians tends to cultivate an Antinomian Spirit. The exclusive inculcation of duties fosters self-righteousness. The edification of the Church, then, demands the diligent intermixture of both kinds. This precept may be confirmed by the remark, that, as the motives and obligations of all duties are rooted in the doctrines, so the best illustrations of the doctrines are by their application to the duties. The two are inseparably connected as grounds and conclusions, as means and end; and their systematic separation in your instructions would leave your hearers incapable of a correct understanding of either.
But the crowning argument is again the precedent set us by Christ and his apostles. While they were, as has been remarked, doctrinal, they were eminently practical preachers. Nothing can be more instructive than the manner in which the Epistles to the Romans, the Galatians, the Ephesians, the Colossians, and the Hebrews proceed. In their introductory chapters, they lay a solid foundation of argument and testimony for some cardinal doctrines of redemption; and from these, they glide into the enforcement of duties by a beautiful transition. The pastor has here models given by inspiration, and obviously conformed to the nature of man as a reasonable and moral creature.
Some important observations remain, touching the mode in which the law should be preached. "We know that the law is spiritual, but we are carnal, sold under sin." The apostle tells us that he had not been made aware of his own concupiscence, except the law had said, "Thou shalt not covet." It is imperative, therefore, that you so unfold the law of God, as to exhibit its searching requirements of right thoughts and feelings, as well as right actions of the bodily members "As a man thinketh in his heart, so is he." "From within, out of the heart of man, proceed evil thoughts, adulteries, fornications, murders." The disease of sin is never so probed as to lead the sufferer in earnest to the Great Physician, until the seat of the evil is revealed in the heart itself. And it is chiefly by disclosing the spirituality of the law, that we affect the convicted soul with a suitable apprehension of the breadth of the law, of his own enmity and inability, and of the infinite holiness of the Author, at once.
Next, let the claims of the law be always enforced, not as moral observances only, but as evangelical duties. If you suppose that, by calling this class of sermons the ethical, I designed to recommend your founding your appeals to men's consciences on the fitness of things, on the natural claims and advantages of virtue, I have been much misunderstood. It is only the morality of the cross which the Christian pastor should teach. It will not be amiss, indeed, for him to show, transiently, how completely the best teachings of the natural morality are at one with those of the gospel. But his chief motives should always be drawn from the latter. Let him inculcate virtue, not like a Seneca, but like a Paul. He should trace every precept of the law to its connection with the redeeming love of Christ, and draw thence his incitements to obey. Would he urge, for instance, Christian fidelity on parents? He will not content himself with appealing to the law of nature expressed in the instinctive parental love, with arguing from the feebleness, dependence or loveliness of our offspring, or with promisng the comforts which dutiful children confer upon our old age. These will be the least of his grounds, and briefly despatched. He will proceed to crown his argument, by directing the hearts of parents chiefly to that Redeemer who claims our children as of his kingdom, to the divine blood with which he has purchased their immortal souls, and to the future of glory and bliss which he offers, chiefly through the means of parental fidelity, to confer on them. Thus, every labour of the father for his child is connected with the Christian's constraining principle -- the love of Christ.
It is a precept of prime weight, that your enforcements of evangelical duty and charges of shortcoming be definite, and even specific. There is, I apprehend, in the pulpits of our Church, no lack of general declarations concerning man's depravity, transgression and guilt. Nor do we find, among the ungodly, any backwardness in making the general confession that they are sinners. But this vague sense of sin and guilt is manifestly without effect. They confess, and still trangress. They avow, in words, their need of cleasning and justification, and yet refuse the salvation offered, with all the supineness of conscious security. It is to be feared that a quickening of these dead hearts will never be effected by launching at them the commonplaces of theology. The mere statements of their responsibility and guilt, in general, will be inadequate. But if their own duties and delinquencies were brought home to them in their details, they would, with the blessing of the Holy Ghost, be made to feel wherein they were sinners indeed, and why under the curse. My meaning may be explained by the instance I employed above. Tell your unrenewed hearer that he is a parent, that he owes duty to his child, and he will readily admit it. Charge shortcoming on him in this duty: he will admit this also, and after the admission he will be as callous as before. But now let us suppose the parental duties defined, and enforced from their high, evangelical obligations, and the cruelty of that parental neglect, which usually destroys the soul of the child, justly painted in the lights of the eternal world. May we not hope that the delinquent parent will acquire some definite conviction of his sin, and especially that his eyes will begin to open to the enormity and malignity of that state of heart charged upon him by the Scriptures, and hitherto so firmly disbelieved by him? In this view of practical preaching, we have a powerful argument for its employment to lead sinners to the Saviour. It is only when we become specific, and apply the general principles of evangelical duty, with close discrimination, to the circumstances of our hearers, that we make the law their "schoolmaster to lead them unto Christ." While we are instructing God's people in the details of their duty, we may be teaching his enemies the number of their sins.