You will have much from the sermon that can serve as lines that reflect Edwards' direct purpose. Edwards did not obscure his sermon with lines that did not hit upon his major ideas of individual sin, God's anger, and how redemption is only possible with complete submission to the divine. I offer a line, but there could be quite a few from the sermon that can represent its direct purpose:
There is nothing that keeps wicked men, at any moment, out of Hell, but the mere pleasure of God.
I think that this quote can represent the direct purpose of the sermon in a couple of ways. The first is that it makes the point that human beings are creatures who sin. One of Edwards' primary motivation in the sermon is to bring out how a part of human nature is that of sin and transgression, something that has plunged humanity into a predicament where damnation is almost a foregone conclusion. Another element that is critical to the purpose of the sermon is the role of the divine. Edwards brings light to the idea that human beings can only be saved through the force of the divine, something that is seen in the line of "mere pleasure of God." In this, another one of the major points of the sermon is revealed in that God is in control of the condition of humanity. I think that the quote is representative of the sermon's purpose in that it highlights an urgency in which human beings must recognize God's awesome powers or suffer the grave consequences. This line helps to deliver the overall purpose of the sermon.