Destructive Power of Such a Small Thing
James 3:5 (ESV) … “So also the tongue is a small member, yet it boasts of great things. How great a forest is set ablaze by such a small fire!”
The images of horses and ships, bridles and rudders show how such small things as the tongue are the real instruments of control over large things. Control is often concentrated at a single, highly significant point in the chain of action: from desire, to movement of the body, and then to fulfillment of deed. James’s attention was on the effective influence of the tongue over the body through the immensity of its boasting. The tongue (cf. 1:26; 3:5–6, 8) is now personified as a self-conscious actor. The tongue, not the self, is the origin of action.
The facts about bits and rudders illustrates the moral truth about the tongue’s control of the body, for the tongue is the great lord of behavior. The tongue might rightly boast of itself, if it could. In spite of its smallness in the body, the tongue not only gives the body its chief means of communication; the body also owes its successes, whatever they are, to the effectiveness of speech. Although the boasting referred to here is not empty, speech really does control the body, a theme repeated throughout the letter. The theme of self-deception has one of its secrets exposed here. Since the believers easily forget how powerful speech is, James was intent upon alerting them. Alertness to the power of speech interrelates to alerts about mixing faith with worldly thinking and desires that undermine faith.
The second half of the verse illustrates the power of speech by comparing the tongue to a spark in a forest. How deceptively small but overwhelmingly dangerous is the little spark. How great,15 how destructive, the fire it kindles in the forest. The large horse controlled by the bit represented the power of desire moved one way or the other by the effectiveness of a small instrument. Because of the control exerted by the tongue, its immense boastfulness must be brought under control. The comparison of the tongue with the spark is then more precise. This analogy was perhaps most useful for James in that he then could call the tongue a fire in the next verse. The little spark kindles a fire of great force.
This illustration is also more complex than those of bit and rudder. The spark is not a neutral metaphor; the destructive figure is unmistakable. A similar use of this example can be found in Philo: “Nothing escapes desire …; like a flame in the forest, it spreads abroad and consumes and destroys everything.” Very likely, then, James meant to say that the boasting the tongue might rightly do must be understood in light of its destructiveness in much of life. 
 Richardson, K. A. (1997). James (Vol. 36, pp. 151–152). Nashville: Broadman & Holman Publishers.