Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
The epigraph is a Bible verse from the Old Testament book of Jeremiah, Chapter 17. It's all about how God will punish the wicked. This particular verse, though, is about the deception and wickedness that every single person has inside.
It would seem that we don't know our own hearts, or what we're capable of. In Other Voices, Other Rooms, the characters all seem to have deep, socially unacceptable desires. Randolph likes to dress up like a lady; Miss Amy kills birds and lies for Randolph; Idabel wishes to be a boy.
And Joel? Maybe this epigraph is a reference to Joel's confusion about what he wants, and whether he'll be allowed to fulfill his longing.