In the sermon "Sinners in the Hands of an Angry God," Jonathan Edwards uses many images to frighten his audience in hopes of persuading them to reform their ways. He believes that they all deserve to be damned and that they will be unless they reform.
Edwards spends a great deal of the sermon emphasizing how angry God is at all the sinners of the world, and especially those in the congregation. However, if one must choose a "key" image, it would probably be that in which Edwards talks about God holding human souls by "a slender thread" over the fires of hell, ready to cut the thread and let them go into eternal damnation.
O sinner! Consider the fearful danger you are in: it is a great furnace of wrath, a wide and bottomless pit, full of the fire of wrath, that you are held over in the hand of that God, whose wrath is provoked and incensed as much against you, as against many of the damned in hell. You hang by a slender thread, with the flames of divine wrath flashing about it, and ready every moment to singe it, and burn it asunder; and you have no interest in any Mediator, and nothing to lay hold of to save yourself, nothing to keep off the flames of wrath, nothing of your own, nothing that you ever have done, nothing that you can do, to induce God to spare you one moment.
The previous post lucidly addresses a critical component of the fear that Edwards invokes. I would like to suggest that Edwards utilizes the imagery of the "now" moment as part of his motivation. He is quite intent on suggesting that part of his rationale in the explanation of God's anger refers to the timing of Colonial sentiments. At a particular moment in time when the colonists are driven by economic prosperity and material wealth, the belief in spirituality is on the decline in colonial life. It is this precise moment that galvanizes Edwards to speak his notion of the spiritual truth relating to the notion of salvation and damnation. It is at this particular moment in Colonial life where God's bow and arrow are set on the Colonists and the sooner they change their ways from the secular to the spiritual, the greater the chance that God's punishment will lessen.
Jonathan Edwards refers to is we are like a spider dangling on the slender thread over a fire which he could just let us drop into the pits of hell.