I believe that those sermons which are fullest of Christ are the most likely to be blessed to the conversion of the hearers. Let your sermons be full of Christ, from beginning to end crammed full of the gospel. As for myself, brethren, I cannot preach anything else but Christ and His cross, for I know nothing else, and long ago, like the apostle Paul, I determined not to know anything else save Jesus Christ and Him crucified. People have often asked me, “What is the secret of your success?” I always answer that I have no other secret but this, that I have preached the gospel,—not about the gospel, but the gospel,—the full, free, glorious gospel of the living Christ who is the incarnation of the good news.[…]There ought to be enough of the gospel in every sermon to save a soul.
Spurgeon, Charles H. The Soul-Winner: or How to Lead Sinners to the Saviour. Kindle Edition, 42-43.