Edwards gave this sermon to his congregation in Enfield, CT, in 1741, during what is known as The Great Awakening in American religion, a period in which leaders of the church were hoping to re-instill religious convictions, which they felt were weakening, in their congregations. It is almost a shame, by the way, that Edwards is chiefly remembered for this harsh sermon--most of his writing and his service to the church was much more positive than this sermon indicates.
The title of the sermon itself--"Sinners in the Hands of an Angry God"--is enough to create fear among people who believe a) that everyone is sinful to some degree and b) that their God is full of wrath rather than peace and light. In their belief system, God is not primarily a merciful being but rather a stern, demanding, angry God who may just as readily toss them into Hell as let them into Heaven.
We know, for example, from accounts of the actual sermon that some people were so shaken by Edwards' sermon that they either fainted or had to leave the church. That alone tells us how seriously they took the threat articulated by Edwards.
One of Edwards' most powerful threats is his reminder that
They are already under a sentence of condemnation to hell. They do not only justly deserve to be cast down thither, but the sentence . . . of God . . . is gone out against them. . . .
Rather than believing they are destined, if they are good and holy, to go to heaven, every listener understands that he or she, as a sinful being, is already destined to end up in hell. We need to keep in mind that hell for these folks is a physical place full of every kind of horrible torture that one can imagine.
Edwards uses very concrete imagery to depict the congregation's dangerous position:
Your wickedness makes you as it were heavy as lead, and to tend downwards with great weight and pressure towards hell; and if God should let you go, you would immediately sink. . . .
The imagery here--lead rolling down and being held up only by God's hand--is meant to convince the congregation that they are literally full of sin and on the verge, by virtue of their weight, of dropping into hell--stopped only by God's hand for the moment. For people who believe in this angry God, as well as their own sinfulness, this is an incredibly perilous situation over which they have no control. Their total lack of control over their fate may be the most powerful and frightening concepts in Edwards sermon.