This comes from John Piper's talk on George Whitefield at the Bethlehem Conference for Pastors earlier this year. First is a quote from Whitefield during a sermon delivered. The second portion is Piper's comment on Whitefield's words.
“I’ll tell you a story. The Archbishop of Canterbury in the year 1675 was acquainted with Mr. Butterton the [actor]. One day the Archbishop . . . said to Butterton . . . ‘pray inform me Mr. Butterton, what is the reason you actors on stage can affect your congregations with speaking of things imaginary, as if they were real, while we in church speak of things real, which our congregations only receive as if they were imaginary?’ ‘Why my Lord,’ says Butterton, ‘the reason is very plain. We actors on stage speak of things imaginary, as if they were real and you in the pulpit speak of things real as if they were imaginary.’”
“Therefore,” added Whitefield, ‘I will bawl [shout loudly], I will not be a velvet-mouthed preacher.”
This means that there are three ways to speak. First, you can speak of an unreal, imaginary world as if it were real—that is what actors do in a play. Second, you can speak about a real world as if it were unreal—that is what half-hearted pastors do when they preach about glorious things in a way that says they are not as terrifying and wonderful as they are. And third is: You can speak about a real spiritual world as if it were wonderfully, terrifyingly, magnificently real (because it is).