What is a sermon and why study it? These are two of the fundamental questions a person interested in sermon studies must answer. Without much reflection, the answer to the first question seems obvious. A sermon is “a discourse, usually delivered from a pulpit and based upon a text of Scripture, for the purpose of giving religious instruction or exhortation,” as the OED puts it.
And yet that simple definition suggested by the OED is not enough. Labeling something “a sermon” ought not to qualify it automatically as one; conversely, it is certainly the case that lacking the label “a sermon” does not preclude a religious discourse from being considered a sermon. And it is the gap between what is definitely a sermon and what may be a sermon that makes the study of the sermon, and preaching, so interesting.
Francis, Keith A.. "Introduction: "Preaching and the Sermon"." Sermon Studies 3.1 (2019) : 1-4. https://mds.marshall.edu/sermonstudies/vol3/iss1/1