John Witherspoon, in his Lectures on Eloquence, with wise words on the importance of preaching with simplicity:

Another character which should distinguish pulpit eloquence is simplicity.

Simplicity is beautiful everywhere; it is of importance that young persons should be formed to a taste for it and more disposed to exceed here than in the opposite extreme, but if I am not mistaken it is more beautiful and the transgressions of it more offensive in the pulpit than any where else. If I heard a lawyer pleading in such a style and manner, as was more adapted to display his own talents than to carry his client’s cause, it would considerably lessen him in my esteem, but if I heard a minister acting the same part I should not be satisfied with contempt, but hold him in detestation.

There are several obvious reasons why simplicity is more especially necessary to a minister than any other.

(1) Many of his audience are poor ignorant creatures.

If he means to do them any service he must keep to what they understand, and that requires more simplicity than persons without experience can easily imagine. It is remarkable that at the first publication it was a character of the gospel that it was preached to the poor. In this our blessed master was distinguished both from the heathen philosophers and Jewish teachers, who confined their instructions in a great measure to their schools, and imparted what they esteemed their most important discourses to only a few chosen disciples.

(2) Simplicity is necessary to preserve the speaker’s character for sincerity.

You heard before how necessary piety is, which is proper parent of sincerity, in the pulpit. Now it is not easy to preserve the opinion of piety and sincerity in the pulpit when there is much ornament. Besides the danger of much affected pomp or foppery of style, a discourse very highly polished even in the truest taste is apt to suggest to the audience that a man is preaching himself and not the cross of Christ.

So nice a matter is this in all public speaking that some critics say that Demosthenes put on purpose some errors in grammar in his discourses that the hearers might be induced to take them for the immediate effusions of the heart, without art, and with little premeditation. I doubt much the solidity of this remark or the certainty of the fact, but however it be, there is no occasion for it in the case of a minister, because preparation and premeditation are expected from him, and in that case he may make his discourses abundantly plain and simple without any affected blunders.

(3) Simplicity is also necessary as suited to the gospel itself, the subject of a minister’s discourses.

Nothing (is) more humbling to the pride of man than the doctrine of the cross; nothing (is) more unbecoming that doctrine than too much finery of language. The apostle Paul chose to preach “not with the words which man’s wisdom teaches” (1 Cor. 2:13)—and again, “not with excellency of speech or of wisdom” (1 Cor. 2:1), which though I admit that it does not condemn study and sound knowledge, yet it certainly shows that the style of the pulpit should be the most simple and self-denied of any other.