Find the direct statment in which Edwards sets forth the purpose of his sermon.

Here is the most direct statement I can find in the entire sermon in terms of specifically stating his purpose:

I am dwelling on this awful subject so that unsaved people in this congregation might be awakened.

As you may know, this sermon is studied because of its great metaphors, similes, persuasive techniques and ability to demonstrate the Puritan belief system. The entire sermon demostrates the wrath and grace of God. About 90% focuses on wrath, 10% on grace at the end. He does a great job demonstrating people are not much on their own until they accept Christ, then all is forgiven.

Here's another pretty direct statement as a good second place for you:

How awful is the condition of those who are daily and hourly in danger of this great wrath and infinite misery. But this is the dismal case of every soul in this congregation that has not been born again, however moral and strict, sober and religious they otherwise might be.

Hope that helps.