Sermons should have real teaching in them, and their doctrine should be solid, substantial, and abundant. We do not enter the pulpit to talk for talk’s sake; we have instructions to convey important to the last degree, and we cannot afford to utter petty nothings. Our range of subjects is all but boundless, and we cannot, therefore, be excused if our discourses are threadbare and devoid of substance. If we speak as ambassadors for God, we need never complain of want of matter, for our message is full to overflowing. The entire gospel must be presented from the pulpit; the whole faith once delivered to the saints must be proclaimed by us. The truth as it is in Jesus must be instructively declared, so that the people may not clearly hear, but know, the joyful sound.
No truth is to be kept back. The doctrine of reserve, so detestable in the mouths of Jesuits, is not one whit the less villainous when accepted by Protestants. It is not true that some doctrines are for the initiated; there is nothing in the Bible which is ashamed of the light. The sublimest views of divine sovereignty have a practical bearing, and are not, as some think, mere metaphysical subtleties; the distinctive utterances of Calvinism have their bearing upon everyday life and ordinary experience, and if you hold such views, or the opposite, you have no dispensation permitting you to conceal your beliefs. Cautious reticence is, in nine cases out of ten, cowardly betrayal. The best policy is never to be politic, but to proclaim every atom of the truth so far as God has taught it to you.
-Charles Spurgeon, from Lectures To My Students; chapter 5 on Sermons-Their Matter