Let us remember that this is a perfect example of a persuasive text where the speaker is trying to persuade his audience of the necessity of becoming reborn and starting a life in relationship with Jesus Christ. To answer your question, one of the aspects that makes this sermon so powerful and persuasive is the way in which Jonathan Edwards uses very strong imagery in terms of metaphors and similes to describe the plight of man and how he stands before God. Consider the following simile that Edwards uses to describe the situation man faces:
The God that holds you over the pit of hell, much as one holds a spider, or some loathsome insect over the fire, abhors you, and is dreadfully provoked...
The way that such images create and inspire fear in the audience clearly is at the heart of the persuasive technique used by Edwards. His images hold no punches and are designed to evoke a feeling of terror in his audience to persuade them of the truth of what he says and make them act as he feels they should.