According to the first paragraph, what keeps sinners from falling from hell?

The first
paragraph establishes the theme that Edwards will strike throughout the sermon.  It is the
"grace" of the divine that prevents individuals from "sliding" into the
fires of Hell.  Edwards makes the argument that those who "live under the means of
grace" are prevented from descending into the pit of infernal condemnation.  Individuals
owe their being in the world to the divine.  From the first paragraph of the speech, Edwards
constructs an interesting paradigm.  On one hand, individuals must possess the power to change
their ways, to reform their paths of sin and transgression.  Yet, the final verdict which
prevents them from going into Hell  is one rendered by the divine.  This condition of being in
the world is reflected in the very first paragraph of Edwards' sermon.

It is
in this "cultivation from heaven" where human redemption lies.  While human beings
have the power to change, the prevention of their "slide" is completely determined by
the will of the divine.  When individuals can understand this in light of God's intense anger at
the path of transgression that has been chosen, Edwards feels that individuals will better
understand their fragile condition and pledge themselves accordingly to the will of the
divine:

In this verse is threatened the vengeance of God
on the wicked unbelieving Israelites, that were Gods visible people, and lived under means of
grace; and that, notwithstanding all Gods wonderful works that he had wrought towards that
people, yet remained, as is expressed, ver. 28. void of counsel, having no
understanding in them; and that, under all the cultivations of heaven, brought forth bitter and
poisonous fruit; as in the two verses next preceding the text.

This becomes the fundamental paradigm of both Edwards sermon and the condition of
humanity at this point in their existence.  It is important enough for Edwards to communicate in
the opening of his work and essential enough for humans to grasp.