Why is it so hard to watch what you say? Why does it even matter? (Note that I’ve made some minor edits to structure to make it easier to read online.)
The tongue is “the world of evil.” In the ancient way of thinking, this is not a difficult phrase. The body was the microcosm of the universe. In all its complexity, the human being was a small, self-contained universe, thus the term “microcosmos.” There is a double sense of microcosm here: not only the body in relation to the universe of nature but also the tongue in relation to the universe of wickedness. Thus, contained within the tongue or speech are all the representations of wickedness in the world.
Is a representation of evil, in words that is, the same as the evil itself? Obviously not, but the power of verbal representation is not slight; this James knew full well. Words have the power to elicit action; indeed, the activity of speech itself interprets every other human action. There is no evil act that the tongue cannot tell, let alone initiate.
In the second half of the verse, the tongue is said to direct the body, and its effects are thorough and total. It perverts the whole body of the person when there is a bent toward evil. True religion keeps “oneself from being polluted by the world” (1:27), but evil speech makes this impossible. For each person the evil of the world has its motive force in the tongue.
The evil spreads, however, to all of a person’s outward relations. Just as the course of a rich man’s life proves to be all too fragile and unstable (cf. 1:11b), the individual’s way of life is all too susceptible to the qualities of his speech. The person, once self-perverted by evil speech, becomes part of the larger currents of wickedness in the world. As such, evil speech proves its true nature as an extension of hell itself.
In a most powerful image, the fire that is the little tongue, a little spark causing great fires, has another fire that causes it. Hell has outcroppings in this world, and one of them is evil speaking.
Three causative relations are laid out here: corrupt speech spawns corruption of the body; the corrupted body sets in motion the evil course of an entire life; the destructiveness of evil speech is derived from the destructiveness of hell.
Since the tongue is the world of evil, the person of perverse speech fails to distinguish between confessing faith and hypocrisy, respect and flattery, blessing and cursing (cf. v. 10). Whoever fails this way in speech certainly will fail in actions.
Evil desire (1:14–15) corrupts the body; and just as a parasite destroys a host organism, the evil tongue becomes parasitical upon the whole life of the individual and indeed the church itself. When it is bent on evil, the tongue is not only its own source of evil but derives some of its inspiration from the great demonic underground.
This connection should not be surprising since in the previous chapters to promote a profession of faith while rejecting active faith is comparable to the “faith” of demons (2:19). The destructive, lying ways of the devil were well known to the biblical writers.
The truth for James was that in the destructiveness of evil speech, the destructive end of that evil was present from the beginning. The destructive force of evil speaking is comparable to the destructive force of hell.
-from Kurt A. Richardson, James (vol. 36; The New American Commentary; Nashville: Broadman & Holman Publishers, 1997), 153–154.