" Unity of discourse requires, then, not only singleness of a dominant subject, but also singleness of practical impression. To secure the former, see to it that the whole discussion may admit of reduction to a single proposition. To secure the latter, let the preacher hold before him, through the whole preparation of the sermon, the one practical effect intended to be produced upon the hearer’s will. "

― Robert Lewis Dabney , Evangelical Eloquence: A Course of Lectures on Preaching