In his book, Christian Leaders of the Last Century, J.C. Ryle details that which made Whitefield’s preaching so effect.
Whitefield preached a singularly pure gospel. Few men, perhaps, ever gave their hearers so much wheat and so little chaff. He did not get up to talk about his party, his cause, his interest or his office. He was perpetually telling you about your sins, your heart, Jesus Christ, the Holy Ghost, the absolute need of repentance, faith, and holiness, in the way that the Bible presents these mighty subjects. ” Oh, the righteousness of Jesus Christ!” he would often say; ” I must be excused if I mention it in almost all my sermons.” Preaching of this kind is the preaching that God delights to honour. It must be pre-eminently a manifestation of truth.
Second, Whitefield’s preaching clear and understanding; as the Puritans would say, his preaching was “plain”. Plain is now a poor word to describe such preaching. “Plain” means to us “dull.” Yet, when one proposes a matter of life and death importance, “plain” is riveting. If a police office bursts into the room, gun drawn and you hear shots being fired, his simple and plain, “Get Down!” Will command your attention. If the policeman were to stop and talk about “root causes of crime” and such, he will merely frustrate you.