What main point does Edwards want his listeners to understand?

First of all Edwards says in his sermon the reasons why he chose the text " their foot shall slide in due time"

1. 
   

   
   

   That they were always exposed to destruction; as one that stands or walks in slippery places is always exposed to fall. This is implied in the manner of their destruction coming upon them, being represented by their foot sliding. The same is expressed, Psalm 73:18. "Surely thou didst set them in slippery places; thou castedst them down into destruction."

   
   

   
   


2. 
   

   
   

   It implies, that they were always exposed to sudden unexpected destruction. As he that walks in slippery places is every moment liable to fall, he cannot foresee one moment whether he shall stand or fall the next; and when he does fall, he falls at once without warning: Which is also expressed in Psalm 73:18,19. "Surely thou didst set them in slippery places; thou castedst them down into destruction: How are they brought into desolation as in a moment!"

   
   

   
   


3. 
   

   
   

   Another thing implied is, that they are liable to fall of themselves, without being thrown down by the hand of another; as he that stands or walks on slippery ground needs nothing but his own weight to throw him down.

   
   

   
   


4. 
   

   
   

   That the reason why they are not fallen already and do not fall now is only that God's appointed time is not come. For it is said, that when that due time, or appointed time comes, their foot shall slide. Then they shall be left to fall, as they are inclined by their own weight. God will not hold them up in these slippery places any longer, but will let them go; and then, at that very instant, they shall fall into destruction; as he that stands on such slippery declining ground, on the edge of a pit, he cannot stand alone, when he is let go he immediately falls and is lost.

   
   

   
   

Therefore as the text outrightly says, their time will come to fall. According to Edwards,it is God's will for them to fail but not yet. He presupposes God to be angry and vengeful.

He wants his listeners to realize that they will fail without God's help and will to prevail. He wants them to understand that they are totally dependent upon God. He envisions God as jealous, in the biblical sense, that he wants to be seen as all powerful and all controlling. When humans act powerful or prideful, God will "take them down a peg" as a way of speaking, to show his power. This is the old fashioned, biblical way of thinking of God, as a jealous God.