The light of Israel. A new name of God. The idea on which it is based may be found in the Psalms (Psalms 27:1; Psalms 84:11), and again in Isaiah (Isaiah 60:19). God enlightens his people, cheers them, comforts them spiritually, as the light of the sun enlightens, cheers, and comforts men physically.
Christ, as true God, is "the true Light, which lighteth every man that cometh into the world" (John 1:9). Shall be for a fire. As the same material fire which gives light, warmth, and comfort may burn and destroy, so the spiritual light, finding fit material, scorches and consumes.
The fire which devours Assyria is to be kindled by God. His Holy One; i.e. "the Holy One of Israel" (see Isaiah 1:4). It shall burn and devour his thorns and his briers. The destruction of Assyria shall resemble that of Israel, in which Assyria was the instrument (Isaiah 9:18).
It shall be as complete, as terrible, and as final. In one day. Scarcely "in one battle" (Cheyne); for the destruction of Assyria was effected by many battles, many sieges, and much exhausting ravage.
"In one day" rather means "at one and the same time," "within a brief space." It is not to he taken literally.