The ongoing unfolding of God’s word that takes place in the relationship of pastor and people is the essence of the sermon.
A sermon is not a sermon unless eternal truth is communicated with the purpose of God’s people responding to the call on their lives, and that those who are not God’s people can understand and heed that same call.
A sermon exists in and for its purpose. That purpose is the persuading and moving men’s souls. That purpose must never be lost sight of. If it ever is, the sermon flags. It is not always on the surface; not always impetuous and eager in the the discourses of the settled pastor as it is in the appeals of the evangelist who speaks this once and this once only to the men he sees before him. The sermon of the habitual preacher grows more sober, but it never can lose out of it this consciousness of a purpose; it never can justify itself in any self-indulgence that will hinder or delay that purpose. It always aimed at men. It is always looking in their faces to see how they are moved..
Phillips Brooks, The Joy Of Preaching, Kregel Classics, 1989, pg. 92.