It is not surprising that Edwards should include biblical quotes and allusions in a sermon. But what specific purpose do these references fulfill, beyond providing more hellish imagery? For Christians, the Bible is a holy book; and most Christians (including those Edwards preached to) believe that the Bible is the divine word of God. This lends the Bible an immense amount of authoritative weight. Even for audience members who might have fallen out of religious practice or belief, the Bible would likely have retained a special spiritual significance. Bible verses thus serve as "proof" in sermons, Edwards's included.
Edwards uses biblical backing to strengthen his argument in many different ways: he analyzes full passages, briefly quotes a section to prove a point, simply references a chapter and verse, or just alludes to a well-known story. While Edwards pulls from the Bible several times in his sermon, we can look at one specific quote that he takes some time to explain. Remember that Edwards's epigraph for his sermon was a Bible verse:
Deut. 32. 35. Their foot shall slide in due time.
Edwards explains this verse at the beginning of his sermon:
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’s coming upon them, being represented by their foot sliding. The same is express’d, Psal. 73. 18. Surely thou didst set them in slippery places; thou castedst them down into destruction.
Edwards lingers for a moment on the verse from Deuteronomy, then adds additional proof by mentioning a verse from the Psalms. Here he establishes an interpretation of these verses: the destruction of the wicked, of God's enemies, is always bound to happen, just as someone standing in an unsteady place will always fall eventually. The image of a sliding foot is useful for Edwards's persuasive purposes here, as it gives the audience visual imagery for their precarious position.
But also note how Edwards can simply say something is from the Bible without needing further justification or explanation. Because Protestants valued a personal connection to the Bible as well as to God, many of them could read and consult Bibles for guidance or study. Edwards mentions the chapter and verse so listeners can verify what he says in their own Bible; it's his way of citing his sources, a practice which lends credence to an argument.
To emphasize the spiritual danger his audience is in, Edwards often relies on imagery of a hungry hell or devouring devils, as seen in this passage:
The Devils watch them; they are ever by them, at their right hand; they stand waiting for them, like greedy hungry lions that see their prey, and expect to have it, but are for the present kept back; if God should withdraw his hand, by which they are restrained, they would in one moment fly upon their poor souls. The old serpent is gaping for them; hell opens its mouth wide to receive them; and if God should permit it, they would be hastily swallowed up and lost.
To be eaten by anything is unpleasant, let alone eaten by a lion or a serpent! There is no escaping a vicious predator that wishes to eat its prey, and here sinners are the prey that vicious animals will tear apart. This image of being eaten is visceral and contributes to the sense of imminent danger Edwards wants to create. Yet, in the midst of this terrifying imagery, Edwards characterizes God as merciful by his "restrain[ing]" of the devils, thereby urging sinners to repent and hasten to Christ without delay.
Also worth noting are the other literary devices Edwards uses to promote this imagery. Although it's not as impressive as his multi-paragraph spider simile, here Edwards writes that the devils which await sinners are "like greedy hungry lions." Likewise, "hell opens its mouth," a personification which allows Edwards to again call up the image of sinners being devoured.
It may seem obvious, and even in Edwards's time it was a common trope, but it is worth noting the fiery imagery Edwards uses to describe hell and God's wrath throughout the sermon, as in this sentence:
The wrath of God burns against them, their damnation don’t slumber, the pit is prepared, the fire is made ready, the furnace is now hot, ready to receive them, the flames do now rage and glow.
Repeatedly, Edwards describes hell as burning hot, and God's wrath is also often compared to fire. This is not a unique association, but it is a visceral image that further builds the audience's terror of hell. More so than today, fires would have been a part of everyday life for nearly everyone alive in this time period. Alongside fires used for cooking and warmth, Edwards's audience might have witnessed larger conflagrations that destroyed homes or whole towns; they might have even seen criminals, disobedient slaves, or accused witches burned at the stake. Edwards's fiery imagery, then, would have connected with the entire audience. Anyone who knew what it was like to be burned, or who had seen people and buildings destroyed by fire, would have then connected God's wrath to that physical feeling of pain they had witnessed or experienced. They would also associate God's wrath and hell with the striking visual of a fire, which Edwards personifies here by saying that the flames "rage."
Finally, note the alliteration Edwards (like many Puritan preachers) employs in the above quote. His list of the ways in which hell is ready to receive sinners is made punchier and more memorable by alliteration.
Edwards not only emphasizes that hell is horrible, but also that it is imminent. In other words, sinners must turn to Jesus with haste, because any day they could die unsaved and go to hell. His choice of metaphor, a bow and arrow, is vividly expounded in the following passage:
The bow of God’s wrath is bent, and the arrow made ready on the string, and justice bends the arrow at your heart, and strains the bow, and it is nothing but the meer pleasure of God, and that of an angry God, without any promise or obligation at all, that keeps the arrow one moment from being made drunk with your blood.
It is worth noting that, like fire, audience members were familiar with bows and arrows. While by this time period most warfare would have been conducted with firearms, Edwards's audience might have hunted with a bow or seen it used in a competition or game.
As a result of that familiarity, and because of the imagery Edwards builds here, the audience would have taken this metaphor as an example of the special effort God exercises in not sending sinners to hell immediately. They also would have understood, once again, that hell was not a far-off problem to be dealt with later, but a potentially immediate consequence. It takes effort to hold an arrow once it is drawn back; the arm and bow alike are tensed. In fact, Edwards mentions this effort explicitly: "justice [...] strains the bow." Once released, arrows are quick and violent. This metaphor emphasizes with accessible visual imagery both the restraint God exercises and the immediacy of punishment once he decides to send someone to hell.