To choose mere moral themes will be to use a wooden dagger; but the great truths of revelation are as sharp swords. Keep to doctrines which stir the conscience and the heart. Remain unwaveringly the champions of a soul-winning gospel. God’s truth is adapted to man, and God’s grace adapts man to it.
There is a key which, under God, can wind up the musical box of man’s nature; get it, and use it daily. Hence I urge you to keep to the old-fashioned gospel, and to that only, for assuredly it is the power of God unto salvation.
Of all I would 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. If with the zeal of Methodists we can preach the doctrine of Puritans a great future is before us. The fire of Wesley, and the fuel of Whitfield, will cause a burning which shall set the forests of error on fire, and warm the very soul of this cold earth. We are not called to proclaim philosophy and metaphysics, but the simple gospel. Man’s fall, his need of a new birth, forgiveness through an atonement, and salvation as the result of faith, these are our battle-axe and weapons of war. We have enough to do to learn and teach these great truths, and accursed be that learning which shall divert us from our mission, or that willful ignorance which shall cripple us in its pursuit. More and more am I jealous lest any views upon prophecy, church government, politics, or even systematic theology, should withdraw one of us from glorying in the cross of Christ. Salvation is a theme for which I would fain enlist every holy tongue. I am greedy after witnesses for the glorious gospel of the blessed God. O that Christ crucified were the universal burden of men of God.
From Lectures to my Students. Lecture V: “Sermons–their matter”