As a highly regarded theologian and preacher, a sermon Jonathan Edwards would deliver would have automatic ethos to his audience in colonial New England. "Sinner in the Hands of an Angry God" is no exception.

In employing ethos in his argument, Edwards makes the point that "the greatest earthly potentates...are but feeble despicable Worms of the Dust, in Comparison of the great and almighty Creator and King of Heaven and Earth" to convince his audience that no mortal man, however powerful, is a match for the omnipotence of God. He goes on to quote the gospel of Luke in asserting "But I will forewarn you whom ye shall fear; fear him, which after he hath killed, hath Power to cast into Hell; yea I say unto you, fear him." The preacher enhances the credibility of his point by invoking the Bible, held to be the sacred text of the people to whom he was delivering the sermon. Edwards's argument in the sermon is that God harbors catastrophic wrath toward sinners. He urges the audience to immediately examine themselves and avail themselves of God's mercy while there is still time. As a theologian and minister, Edwards presents himself as an interpreter of God's expectations for humanity and the man who can lead them to their salvation if they heed his dire warning that eternal damnation is that fate of any man who disobeys God, whether he is a king or an ordinary man. Edwards seeks to convince people that God does not discriminate based on a person's station in earthly life, only his conduct.

Ethos is one of the three elements of rhetoric (the other two being logos and pathos). While pathos is an appeal to the emotions and logos is an appeal to logic, ethos is an appeal to ethics. Ethos specifically highlights the character of the orator.

In any oration or sermon, the speaker's credibility is reinforced when he can provide evidence that highlights (among other things) his flawless reputation and impeccable credentials. Basically, the speaker needs to show that he is qualified to argue his claims. In the sermon you reference, Jonathan Edwards uses a few appeals to ethos to reinforce the necessity of his listeners repenting and being born again (in a spiritual sense).

He quotes readily from the Bible, showing that he is well-versed in the Scriptures and therefore, qualified to dispense spiritual advice to his multitude of listeners. One of the verses he quotes is from John 3:18 "He that believeth not is condemned already." Edwards uses this verse to argue that every unconverted parishioner is in danger of the fires of Hell.

Throughout his sermon, Edwards reinforces the idea of an angry God presiding over the affairs of men. He uses various Bible verses to back up his claim. Edwards maintains that even an earthly king becomes angry when his subjects disobey him. According to Edwards, by way of the divine right of kings, a monarch has the right to expect perfect obedience from his subjects: rebellion in any form is considered to be treason against the king.

From this, Edwards argues that, if an earthly king has the right to expect obedience, then God himself (as a heavenly king) has the right to expect commensurate obedience in affairs of the soul. In fact, Edwards argues that God has paramount claim on the souls of all men.

All the Kings of the Earth before GOD are as Grasshoppers, they are nothing and less than nothing: Both their Love and their Hatred is to be despised. The Wrath of the great King of Kings is as much more terrible than their’s, as his Majesty is greater. Luke 12. 4,5. And I say unto you my Friends, be not afraid of them that kill the Body, and after that have no more that they can do: But I will forewarn you whom ye shall fear; fear him, which after he hath killed, hath Power to cast into Hell; yea I say unto you, fear him.

In keeping with his tendency of using his strong command of scripture to reinforce his claims, Edwards ends his sermon with a biblical allusion: "Haste and escape for your lives, look not behind you, escape to the Mountain, least you be consumed." This exhortation refers to the angel's warning to Lot and his family to flee before God destroys Sodom (Genesis Chapter 19).

Certainly, the rhetoric of "Sinners in the Hands of an Angry God is most associated with pathos. Edwards repeatedly portrays people as inherently sinful, always precariously on the edge of destruction at the hands of a God who would be more than justified in allowing them to be consumed by the flames of Hell. 

But Edwards also makes an argument from ethos. He does this by appealing to perhaps the most important and persuasive authority in the minds of New Englanders: the Bible. The sermon is based on a passage from Deuteronomy that reads, "Their feet shall slide in due time." He then proceeds to lecture on the meaning of this passage, which gives scriptural authority to his overall message. He explains that this passage refers to the threat of God's vengeance on "the wicked, unbelieving Israelites" who were always subject, like the people of New England, to God's wrath. Edwards concludes before moving on to the main thrust of his sermon: 

The observation from the words that I would now insist upon is this: "There is nothing that keeps wicked men at any one moment out of hell, but the mere pleasure of God.

So Edwards sought to establish authority and expertise (ethos) by basing his sermon on the authority of the Bible.