To engage the heart is also to engage the imagination, and the imagination is more affected by images than by propositions. Here, we are talking about what are usually termed “sermon illustrations.” Over the past generation, preachers have recaptured the importance of story. Common sense tells us that stories capture interest and stick in the mind, and so preachers are often advised to lace their sermons liberally with narratives. However, we should think a bit more deeply about why they are so effective. An illustration is anything that connects an abstract proposition with the memory of an experience in the sensory world. Here, again, we can learn much from Jonathan Edwards. Edwards knew well the preacher’s great challenge – that people could subscribe to many propositions of Christian doctrine with their minds that did not influence how they actually lived their lives. Why?
He argued that human beings are body-bound creatures and, because of our fallenness, spiritual realities are simply not as real to us as sense experience – things we actually see, hear, touch, smell, and taste. Objects that we can experience through our senses are real to us – they are memorable and make impressions that last. While people can agree that “abstractions are true. . .Only images [things they have experienced with the senses] seem real.”
Timothy Keller: Preaching: Communicating Faith in an Age of Skepticism, pp.169-170.