We're not sure who is narrating, but he or she wonders if free will is "the greatest gift bestowed upon humanity" (Prologue.2) or "a crap myth" (Prologue.3).
What our narrator believes is that "choice is our free will" (Prologue.5), meaning that your fate is always going to be the same, but you can choose how you eventually get there.
To close this philosophical prologue, our narrator says he "curse[s] the day I ever called Acheron Parthenopaeus my friend" (Prologue.13) because friends don't let friends have such unpronounceable last names.
Also, he says, "I am Nick Gautier. And this is my life and how things should have been…" (Prolgoue.14-Prologue.15).
Here we go. It's going to be like the 50s TV show This Is Your Life—but with more demons.