In rereading James recently I wondered if our speech is the integrating theme of really everything James says, even though he seems to hop from one topic haphazardly to the next. Throughout, James gives the impression: what we say isn't an aspect of what defines us and what kind of religion we have, but comprehends the whole.
So as I am now in Luke I was struck by the way Jesus concludes his teaching on a tree being known by its fruit in Luke 6. I have always read this tree/fruit thing to be a statement about our deeds, practically speaking. Just as a healthy tree produces healthy fruit, a healthy heart produces healthy deeds.
But notice what Jesus actually says: "The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks" (Luke 6:45).
We are defined by our words. Our words are who we are. I don't mean identity-definition in a gospel sense, our identity as sons and daughters of God, in defiance of our word-failings. A less ultimate but no less comprehensive identity: what I say is who I am. I simply have nothing else. Maybe that's why James says that whoever can tame the tongue is perfect, complete (James 3:2).