5 — Behold, how great a matter a little fire kindleth!
5 Ecce exiguus ignis quantam sylvam incendit.
6 And the tongue is a fire, a world of iniquity: so is the tongue among our members, that it defileth the whole body, and setteth on fire the course of nature; and it is set on fire of hell.
6 Et lingua ignis est, et mundus iniquitatis: sic inquam lingua constituta est in membris nostris, inquinans totum corpus, inflammans rotam nativitatis, et inflammatur a gehenna.
He now explains the evils which proceed from the neglect of restraining the tongue, in order that we may know that the tongue may do much good or much evil, — that if it be modest and well regulated, it becomes a bridle to the whole life, but that if it be petulant and violent, like a fire it destroys all things.
He represents it as a small or little fire, to intimate that this smallness of the tongue will not be a hindrance that its power should not extend far and wide to do harm.
6. By adding that it is a world of iniquity, it is the same as though he had called it the sea or the abyss. And he suitably connects the smallness of the tongue with the vastness of the world; according to this meaning, A slender portion of flesh contains in it the whole world of iniquity.
So is the tongue. He explains what he meant by the term world, even because the contagion of the tongue spreads through every part of life; or rather he shews what he understood by the metaphor fire, even that the tongue pollutes the whole man. He however immediately returns to the fire, and says, that the whole course of nature is set on fire by the tongue. And he compares human life to a course or a wheel: and genesis, as before, he takes for nature, (James 1:23.)
The meaning is, that when other vices are corrected by age or by the succession of time, or when at least then do not possess the whole man, the vice of the tongue spreads and prevails over every part of life; except one prefers to take setting on fire as signifying a violent impulse, for we call that fervid which is accompanied with violence. And thus Horace speaks of wheels, for he calls chariots in battle fervid, on account of their rapidity. The meaning then would be, that the tongue is like untamed horses; for as these draw violently the chariots, so the tongue hurries a man headlong by its own wantonness. 
When he says that it is set on fire by hell, it is the same as though he had said, that the outrageousness of the tongue is the flame of the infernal fire.  For as heathen poets imagined that the wicked are tormented by the torches of the Furies; so it is true, that Satan by the fans of temptations kindles the fire of all evils in the world: but James means, that fire, sent by Satan, is most easily caught by the tongue, so that it immediately burns; in short, that it is a material fitted for receiving or fostering and increasing the fire of hell.
 "The course of nature," or the compass of nature, that is, all that is included in nature, means evidently the same with "the whole body" in the preceding clause. There is no sense, compatible with the passage, in what some have suggested, "the whole course of life;" for what idea is conveyed, when we say that the tongue inflames or sets in a flame the whole course of life? But there is an intelligible meaning, when it is said, that the tongue sets in a flame the whole machinery of our nature, every faculty that belongs to man.  "A bad tongue is the organ of the devil." -- Estius.
 "A bad tongue is the organ of the devil." -- Estius.