It is not surprising that you would have trouble finding examples of ethos in “Sinners in the Hands of an Angry God.” This sermon is based much more on pathos than on ethos. Edwards does not say anything about his own qualifications. He does not offer any personal testimony about how he has felt God’s mercy. Instead, the examples of ethos in this sermon are to be found in his numerous citations of Bible verses.
When a speaker or writer uses ethos, they are trying to persuade by showing that they are credible. They are saying we should believe them because they know what they are talking about. In this sermon, Edwards proves that what is saying is true by citing many Bible verses. In Edwards’ time and place, many people believed that the Bible was the ultimate source of authority. Therefore, if you could say “look, the Bible agrees with me and that proves I know what I’m talking about,” you would be able to persuade many people. This is part of what Edwards does to persuade in this sermon.
As far as quotes go, any passage in which Edwards cites a Bible verse will do. You might want to simply use the quote “Their foot shall slide in due time,” since that is the basis for Edwards’ argument.