Here’s how C.H. Spurgeon summarized what he believed the matter of a sermon should be:
Of all I wish to say this is the sum; my brethren, preach Christ, always and evermore. He is the whole gospel. His person, offices, and work must be our one great, all-comprehending theme. The world needs still to be told of its Saviour, and of the way to reach him. Justification by faith should be far more than it is the daily testimony of Protestant pulpits; and if with this master-truth there should be more generally associated the other great doctrines of grace, the better for our churches and our age.
From Spurgeon’s Lectures to My Students (Chapter 5: “Sermons — Their Matter”).