Therefore, etc. A general judgment is now pronounced against all the forms of wickedness enumerated—a judgment of ruin or destruction. It is expressed by a mixed metaphor, or "combination of two figures," the former taken from the burning of stubble and withered grass by the farmer when he is cleaning his fields, the latter from the natural decay of a blossoming plant or tree.
In either case the destruction is complete, but in the one it arises from an external force, fire; in the other from an internal failure of vitality. The ruin of Israel would include both; it would be brought about by an internal cause, their corruption, and an external one, God's anger.
As the fire devoureth the stubble; literally, as a tongue of fire eats up stubble. "Tongue of fire" is an unusual phrase, occurring in all Scripture only here and in Acts 2:3. But it well depicts the power of fire to lick up clean all that comes in its way.
Isaiah elsewhere notes the analogy, making it the foundation of simile (Isaiah 30:27). And the flame consumeth the chaff; rather, and as dry grass sinks down inflame. The withered grass of pastures was burnt by farmers to improve the after-growth (Lucan, 'Pharsal.
,' 9.182). Their root shall be as rottenness (comp. Hosea 9:16). The root is the last thing to decay. When that fails, the case is desperate. Judah's "root" did not utterly fail (see Isaiah 11:1); but the present warning is to individuals and classes (verses 8, 11, 18, 20-23), not to the nation.
Their blossom shall go up as dust; i.e. their external glory shall crumble and waste away. Because they have cast away the Law. All the sins of Israel had this one thing in common—they were transgressions of the Law of God as delivered to them by Moses, and enforced upon them by the prophetical order.
Despised the word; or, the speech. Imrah is rarely used by Isaiah. It does not refer to the written "Word," but to the declarations of God by the mouth of his prophets (see Isaiah 28:23; Isaiah 32:9).