According to Jonathan Edwards in "Sinners in the Hands of an Angry God," what is the only reason one of these unsaved men is not currently burning in hell?

According to Jonathan Edwards in “Sinners in the Hands of an Angry God,” the only reason that unsaved men (or anyone else) have not been cast down into hell to burn eternally is God’s mercy. These people are only saved from damnation by God’s will.
In this sermon, Edwards is trying to emphasize to his listeners that they deserve to be damned. He tells them over and over that their sins are offensive to God. He reminds them that God is very disappointed in them and that he has every right to be. He says that all people “have deserved the fiery pit.” In one of the most famous passages from this sermon, Edwards tells the people that

The God that holds you over the pit of hell, much as one holds a spider, or some loathsome insect, over the fire, abhors you, and is dreadfully provoked; his wrath towards you burns like fire; he looks upon you as worthy of nothing else, but to be cast into the fire; he is of purer eyes than to bear to have you in his sight; you are ten thousand times so abominable in his eyes as the most hateful venomous serpent is in ours.  

This is what Edwards thinks of humanity. He thinks that they (we) all deserve to be cast into hell because we have sinned against God. So why are we not in hell? The only reason for this is that God has, for some reason, chosen not to let us fall. It is not because of anything we have done. We cannot really deserve God’s mercy. God has been good enough to give us that mercy so far, though. As Edwards says,

‘tis nothing but his hand that holds you from falling into the fire every moment; 'tis to be ascribed to nothing else, that you did not go to hell the last night; that you was suffered to awake again in this world, after you closed your eyes to sleep: and there is no other reason to be given why you have not dropped into hell since you arose in the morning, but that God's hand has held you up.

From this, we can see that the only reason that unsaved men and women are not in hell is because God has spared them. God did not have to spare them. People do not deserve to be spared. Instead, it is simply God’s own mercy, freely given, that has prevented unsaved people from being dropped into hell.