I went right along, not fixing up any particular plan, but just trusting to Providence to put the right words in my mouth when the time come; for I'd noticed that Providence always did put the right words in my mouth if I left it alone.—Mark Twain, Huckleberry Finn, Chapter 32
What's up with the epigraph?
The point of the epigraph is pretty clear. This is a book by a nun who often talks about the importance of her faith. Huck Finn says he trusts to Providence to put the right words in his mouth, and Sister Prejean also trusts Providence to put the right words in her mouth: she's hoping that God will speak through her and help her find a way to convince people that the death penalty is wrong.
Now, while this quote is about God, it's not actually the word of God. You might expect a book by a nun would have a Biblical quotation as an epigraph, but instead, Prejean chose a famous secular book.
Perhaps she was thinking of Sonnier's request when he first started talking to her that they "just talk to each other in regular words" (1.58) rather than in Scriptures from the Bible. Prejean wants to convince as broad an audience as possible. Using Twain rather than the Bible may be a way to signal that she's going to talk in "regular words"—she's speaking to everyone she can, not just to Christians.