Edwards begins the sermon saying that God will enact vengeance on those who do not follow his word (counsel). He adds that the mere will of God is the only thing keeping wicked men out of hell. This is God's "arbitrary mercy" that keeps wicked men out of hell. He calls it "arbitrary" to suggest that God's mercy on the wicked is meaningless and He can therefore just as arbitrarily quit showing that mercy at any time. When God decides to cast someone into hell, he shows he is all powerful; there is no stopping Him.
Edwards claims that God is more angry "now" (at the time of his sermon) with people on the earth than he is with all of the people already in hell. He repeats that it is only God's restraining hand that keeps the wicked from slipping into hell. This is to emphasize that the slip can occur at any moment, no matter how healthy or safe a man or woman thinks he or she is. Regardless of the Covenant of Works (God's deal with Adam) or the Covenant of Grace (God's deal with humanity following the Fall), God has made no promises about saving anyone from hell.
In the "Application" section of his sermon, Edwards continues with descriptions of God's wrath. He is intent on waking his listeners up, of shocking them into being more righteous and therefore open to religious experience. He tries to put God's spiritual wrath in physical, earthly terms. God's wrath is much worse than any king's or warrior's oppression and violence. There is no earthly analog that can come close to describing the actual wrath of God.
At the end of the sermon which primarily hammers the points of God's wrath and vengeance, Edwards offers his listeners a glimpse of God's mercy. Those in hell are beyond hope. But those to whom he is preaching, they still have a chance for salvation.
Their case is past all hope; they are crying in extreme misery and perfect despair; but here you are in the land of the living and in the house of God, and have an opportunity to obtain salvation.